CHAPTER V.

MANUFACTURE OF BONE-GLUE.

The manufacture of bone-glue differs chiefly from that of skin-glue in the processes employed for the conversion of the glue-yielding tissues. This conversion may be effected by boiling the bones with water, or subjecting them to the action of steam, or by first extracting their mineral constituents with acid, and boiling the remaining cartilaginous mass with water until dissolved.

When the finest quality of all varieties of glue, namely, colorless gelatine, is to be manufactured, the bones should not be comminuted in a stamping mill, because in consequence of the unavoidable development of heat, they acquire a slightly empyreumatic odor which adheres to the gelatine prepared from them, and cannot be removed.

In factories working on a small scale, the bones are comminuted by hand, being placed upon a grate-like support of heavy iron rods and crushed with a wooden hammer, the face of which is studded with big-headed nails. In larger establishments the crushing rolls previously described are used, and in order to lessen the effect of heating as much as possible, the crushed bones are allowed to fall directly into a vessel filled with water.

Fat being a very valuable constituent of bones, it should be gained as completely as possible, by boiling or steaming the bones, or by extracting them by means of a solvent, such as benzine or carbon disulphide.

1. BOILING BONES.

This is the older and more incomplete process of extracting the fat. The bones are placed in a boiler, covered with water so that it stands a few inches deep over them, and the whole is boiled over an open fire. The melted fat collecting on the surface of the water is skimmed off. By boiling, a portion of the glue-yielding substance is, of course, converted into glue, and passes into the water. In order not to lose this glue, the same water is repeatedly used for boiling fresh quantities of bones, and is finally used for feeding pigs. By this method 4 to 5 per cent. of fat is at the utmost obtained.

The bone fat obtained by direct boiling of the bones, is, if entirely fresh material has not been used, of very inferior quality. It is dark yellow to deep brown and of a disagreeable odor. It is only fit for certain purposes, and to be utilized in the manufacture of soap has to undergo a special process of purification, whereby it is rendered white and odorless.

2. STEAMING BONES.

In order to obtain a larger quantity of fat than is possible by boiling, the bones are preferably steamed, _i. e._, subjected to the action of high-pressure steam. This is effected in a closed cylinder of thick boiler-plate, into which steam of ½ to 1 atmosphere pressure is admitted. The cylinder is provided with a perforated false bottom upon which the bones are placed. By steaming for two or three hours, all the fat is extracted from the bones and collects, together with the condensed water formed by the steam coming in contact with the cold bones, underneath the false bottom. However, by the continued action of high-pressure steam upon the bones, a considerable portion of the glue-yielding tissue is converted into glue, which passes into the resulting liquor. This, however, is no drawback if only fat and glue are to be obtained from the bones, since by continued steaming, a liquor still richer in glue results, and need only be evaporated. But, as a rule, the greater portion of the bones, especially the granulated parts, are to be utilized in the manufacture of animal charcoal, and, hence, great care has to be observed in steaming.

Animal charcoal is produced by calcining bones in vessels from which the air is excluded, whereby the glue-yielding tissue is converted into carbon, which is distributed upon the bone-earth. Since the value of animal charcoal depends on the quantity of carbon it contains, a product prepared from bones highly steamed, will evidently be of little value, as a considerable portion of the glue-yielding substance has been converted into glue.

If the bones are to be used for the production of animal charcoal they should be subjected to the action of high-pressure steam only long enough to extract the fat, but the resulting glue-liquor is very thin and difficult to work. The watery glue-liquor is first drawn off, and the fat which comes last is caught by itself. The thin glue-liquor is evaporated in vacuum.

3. EXTRACTION OF BONES.

To avoid the loss of glue-yielding substance which is unavoidable in steaming bones, even if only for a short time, in many plants the fat is now extracted by treating the bones with benzine or carbon disulphide. No loss of glue-yielding substance being involved by this process, bones thus treated yield the best quality of animal charcoal.

The fat obtained by extraction with carbon disulphide has such a disagreeable odor as to render it almost worthless. In addition this solvent is very volatile, consequently very inflammable, and is also very poisonous. For these reasons its use for the extraction of fat has been almost entirely abandoned.

Figs. 31 and 32 show an apparatus for the use of benzine which is the invention of Messrs. Wm. Adamson and Charles F. A. Simonis, of Philadelphia, Pa. It is for the purpose of treating animal and vegetable substances with hydrocarbons for extracting therefrom oily, fatty and resinous matter; and the object of this invention is to cause hydrocarbons to trickle through such substances instead of flooding the same, so that it will take up the oily, fatty and resinous matter without any of the albuminous or gelatinous ingredients.

Fig. 31 is a vertical section of apparatus wherewith this invention may be carried into effect; Fig. 32, an inverted plan view of part of Fig. 31.

_A_ is a vessel, preferably of cylindrical form, and containing an upper perforated diaphragm, _a_, and lower perforated diaphragm, _b_, the former having a central opening, through which the material to be treated may be introduced between the two diaphragms, and this opening having a detachable perforated cover, _d_.

On the top of the vessel there is an opening, _e_, furnished with a detachable cover, _f_, and at the bottom of the vessel there is an outlet-pipe, _h_, furnished with a suitable cock or valve, _i_.

Liquid hydrocarbon, preferable such as is of a volatile character—benzine, benzole, or gasoline, for instance—is introduced into the vessel above the diaphragm _a_ through a pipe, _H_, and perforated ring, _I_, or otherwise, the hydrocarbon passing through the diaphragm and falls in a shower on the substance contained in the vessel.

The hydrocarbon will trickle through the mass, taking up whatever oily, resinous, or fatty matter it comes in contact with until it falls through the lower diaphragm into the space _D_, whence it may be drawn off from time to time through the outlet-pipe, _h_.

In extracting oily, fatty, or resinous matter from vegetable or animal substances by hydrocarbons, it has been the practice either to subject them to hydrocarbon vapors, or to immerse or steep the substances in hydrocarbon until the latter takes up the oily, fatty, or resinous matter.

The vapor plan is preferable in treating wet animal substances, such as offal; but for dry vegetable or animal matter—seeds, for instance, or the residuum resulting from the rendering of tallow—we prefer the plan before described.

The flooding or steeping of animal or vegetable matter in liquid hydrocarbon results in a mixture or emulsion of gelatinous, albuminous, and fatty or oily matter, combined with animal or vegetable tissues, the whole forming an amalgamated mass; hence, whatever fatty or oily matter is extracted is accompanied by more or less of the suspended gelatine or albumen, either of which is more difficult to remove from the oil or fat, and has a tendency to discolor the same.

This difficulty, it has been found, can be obviated by preventing the hydrocarbon from remaining in a quiescent state in contact with the material; in other words, by causing it to trickle through the mass, which, by this plan, retains its granular condition, and gives out its oil or fat to the hydrocarbon without the albuminous or gelatinous matter.

In the apparatus before described, for instance, an occurrence of the objectionable flooding of the material, tending to bring about the results previously mentioned, is obviated by never permitting the extract in the lower portion of the vessel _A_ to reach the lower diaphragm _b_. By drawing off the extract from time to time, any impediment to the free discharge of the hydrocarbon with such oily and resinous matter as it has taken up, through the lower diaphragm, is prevented, and a continuous dripping of the hydrocarbon through the mass secured.

The extract obtained by the trickling or filtering process is much more concentrated than that obtained by the steeping and flooding process.

_Adamson’s Method for Treating Substances with Hydrocarbon Vapor for the Purpose of Extracting Oils, Fats, etc._ This improvement is intended to prevent the fetid or other odors imparted to the vapor from the substances treated from being recommunicated to the said substances, and to the extracts obtained therefrom through the medium of the vapor from the re-used hydrocarbon. The vapor is obtained from benzine, benzole, etc.

Fig. 33 represents, partly in section, the apparatus whereby the invention may be carried into effect.

_A_ is a vessel in which the substances have to be treated by hydrocarbon vapor, the said substances being introduced into the vessel through a manhole, _x_, and deposited on a perforated diaphragm, _B_, the manhole being provided with a suitable cover. A steam-coil, _D_, is placed in the vessel in a space beneath the diaphragm, and liquid hydrocarbon is introduced into the said space, and is there vaporized by the steam-coil. The vapor rising through the perforated diaphragm permeates the substance upon the same, so as to extract therefrom the oily, fatty, or resinous matter, which passes downward through the diaphragm into the space below the same, whence it may be drawn off from time to time through the discharge-pipe _j_. Liquid hydrocarbon may be introduced from a tank, or from a source explained hereafter, into the top of the vessel _A_, so that it will pass through the material and be vaporized when it reaches the coil; the said material being in this case subjected to a downward current of liquid hydrocarbon and an upward current of vapor.

Previous to this invention it was Mr. Adamson’s practice to cause the vapor, after acting upon the substances in the vessel, to pass through a worm in a condenser, the lower end of the worm communicating with the vessel, _A_, beneath the diaphragm, as shown in Fig. 35, p. 85, so that the hydrocarbon was used over and over again. But in practice this has been found objectionable in many cases for the following reason:—

In treating animal offal, for instance, for the extraction of fats, fetid odors are imparted to the hydrocarbon vapor, and remain, to a considerable extent, in the condensed vapor when the latter is restored to the vessel _A_; hence, the fetid odors were recommunicated both to the fatty extracts and to the material. The same objections have been experienced in treating meat for preservation and vegetable matter for the extraction of oil by hydrocarbon vapor.

This difficulty is obviated in the following manner: The vapor-pipe _D´_ communicates with a vessel _H_ at the top of the same, and the vapor is met by numerous small jets of cold water—in the present instance, from a perforated tubular ring, _m_, into which the water is forced through a pipe _n_.

Many different appliances may be used, such as roses, revolving jets, etc., for causing a spray through which the vapor must pass, and by which it must be condensed. The result of this will be a supply, _I_, of tainted water on the bottom of the vessel, _H_, and a quantity, _J_, of washed and purified hydrocarbon above the water, the latter having taken up the fetid odors.

The washed hydrocarbon may be drawn off through a pipe, _g_, into any suitable vessel, and thence introduced through the pipe _h_ into the vessel _A_, or may pass directly into the latter to be again vaporized therein, the vapor after permeating the material and passing through the pipe _D´_ being simultaneously condensed and washed in the vessel _H_, preparatory to being returned in the condition of purified liquid hydrocarbon to the vessel _A_.

By the practice of this process, the inventor is enabled to obtain a purer extract than heretofore, and, at the same time, the substances acted upon are more free from noxious odors.

Changes may be made in the apparatus shown in Fig. 33, as, for instance, the vessel _A_ may consist of a horizontal hollow cylinder, and the vaporizing of the hydrocarbon may be accomplished otherwise than by a steam-coil.

_Adamson’s Method for Treating Substances with Liquid Hydrocarbon for the Purpose of Extracting Oils, Fats, etc._ This invention relates to a method of treating animal and vegetable substances with liquid hydrocarbons, such as benzene, benzole, etc., for the purpose of extracting from such substances oils, fats, etc.

The object of this improvement is to prevent the fetid and other odors imparted to the liquid hydrocarbon by the substances treated from being recommunicated to the substances and to the extracts therefrom by the liquid hydrocarbon when re-used.

In Fig. 34, there is shown a sectional view of apparatus whereby this invention may be carried into effect.

_A_ is a vessel into which the substances to be treated are introduced through a manhole, _x_, provided with a suitable detachable cover, and through an opening in the upper perforated diaphragm, _B_, a detachable perforated plate, _b_, being placed over the opening after the substances have been passed through the same, the substances being supported by the lower perforated diaphragm, _B´_, beneath which is a space for receiving the extract and liquid hydrocarbon after the latter has percolated through the mass in the vessel. The extract, which occupies the lowest position in the vessel, may be removed therefrom from time to time prior to being purified by distillation or otherwise. The liquid hydrocarbon is permitted to pass from time to time through a pipe, _d_, into a vessel, _D_, where it is met by jets of water from a pipe, _f_, the hydrocarbon and water being thoroughly agitated in the vessel by a revolving paddle-wheel, _E_. This washing of the liquid hydrocarbon may be accomplished by different appliances. For instance, the paddle-wheel may be dispensed with, and water forced upward into the vessel from below in the form of numerous small jets. The water and hydrocarbon after this washing operation are permitted to pass into the subsiding-vessel, _H_, the hydrocarbon being above and the water below, the fetid and other odors divided by the hydrocarbon from the substances in the vessel, _A_, having, during the washing operation, been transferred to the water, which may be drawn off from time to time.

The washed and purified hydrocarbon may be pumped directly through a pipe, _m_, into the vessel, _A_, to be re-used for treating the substances therein; or it may be pumped, first, into a reservoir, and permitted to flow from the same into the said vessel, _A_.

More or less hydrocarbon is wasted by being drawn off with the extract, and to make up for this loss a supply may be introduced at intervals from a tank through the pipe, _h_.

By the practice of the process described above, the inventor is enabled to obtain a purer extract than by the ordinary process of treating substances with liquid hydrocarbon. At the same time the substances treated will be much more free from noxious odors than when the hydrocarbon is used over and over again without washing.

It is not essential strictly to adhere to the apparatus shown in Fig. 34, as the construction of the apparatus will, in fact, depend in a great measure on the locality in which it is to be situated.

_Adamson’s Process for Removing Hydrocarbons from Substances which have been treated therewith._ This process consists of washing from animal and vegetable substances the hydrocarbon which they retain after being treated therewith for the extraction of oils, fats, etc., and for other purposes.

Different apparatus may be employed for carrying out this process, and it may be conducted in the same vessel in which the material is treated with hydrocarbon.

The vessel, which is shown in Fig. 35, has been found to answer well for this purpose.

This vessel is furnished with a suitable detachable cover, _a_, and with two perforated or wire-gauze diaphragms, _b_ and _d_, both extending across the interior of the vessel, one near the top and the other near the bottom of the same.

A steam-coil, _B_, communicating with any adjacent steam-generator, is contained in the vessel below the lower diaphragm, to vaporize the hydrocarbon, the vapor passing through the substance between the two diaphragms and out through a pipe, _D_, which passes through a condenser, _E_, the latter restoring the hydrocarbon to a liquid form, in which it is reconveyed to the vessel through a pipe, _D´_.

In practicing the washing process a pipe, _m_, to introduce water into the vessel, and one or more outlet-pipes, _n n´_, two in the present instance, are necessary. There may also be a pipe, _p_, through which air can be introduced into the vessel, under the circumstances explained hereafter.

When the treatment of the material in the vessel with hydrocarbon vapor or liquid hydrocarbon has been completed, steam is cut off from the coil _B_, the pipes _D_ and _D´_ are closed, and the cover _a_ may be removed.

Water is now admitted through the pipe _m_ to the space in the vessel below the diaphragm _d_, and the cocks of the outlet-pipes _n n′_ are opened.

The water permeates the material, passes upward through the same, and carries with it the hydrocarbon, the latter having a tendency to rise with the water.

As the water, and whatever hydrocarbon accompanies it, pass through the upper diaphragm, _b_, the hydrocarbon will at once rise to the surface, and will pass through the upper outlet-pipe, _n_, into any suitable receptacle, the water passing off through the lower outlet-pipe.

If this mode of separating the hydrocarbon from the water is practiced, the admission of water to the vessel should be such in respect to the outflow that the liquid will remain at or near a uniform level, that is, the surface of the liquid should bear the relation shown in the drawing to the upper outlet.

The water and hydrocarbon, however, may be drawn off indiscriminately into a suitable receptacle, and then separated by decantation; but it is advisable in all cases that the water should extend above the mass of material in the vessel, so that the hydrocarbon can at once rise to the surface as it escapes from the substance.

When the material is of such a character as to be closely packed and not easily displaced by the upwardly-flowing water (and this is especially the case with seeds which have been treated with hydrocarbons), it is necessary to agitate the mass, so that the water can gain access to every part thereof. This agitation the inventor prefers to effect by air under pressure introduced through a pipe, _p_, although mechanical appliances may be used for the purpose.

It will be understood that the process may be conducted in a vessel separate from which the substances have been treated with hydrocarbon. A vessel similar to that shown, for instance, but without the coil and pipes, _D D´_, may be used, and may be furnished with trunnions (shown by dotted lines) and adapted to bearings, so as to be easily tilted when its contents have to be removed; or the vessel may have an opening near the lower diaphragm for the withdrawal of its contents, a suitable detachable door being adapted to the opening.

_F. Seltsam’s apparatus._ In this process the solvent is boiled with the bones, previously coarsely crushed and the dust sifted out, in a strong closed vessel, so as to obtain a higher temperature, greater penetration and avoidance of loss. The vapor ascending condenses in the pores, extracts the fat and collects under the false bottom as a layer of solution which is subsequently distilled. The apparatus is shown in Fig. 36. The cylinder, _A_, is capable of withstanding a pressure of 10 atmospheres, and serves for the generation of steam and as an extracting vessel. It is filled with bones and hermetically closed. The required quantity of solvent is then brought by means of the pump, _B_, from the reservoir, _C_, through the pipe, _D_, into the cylinder, _A_, and the latter is heated. The vapors formed force the air through the pipe, _E_, into the condenser, _F_, where any vapor which may be carried along is condensed and passes through the pipe, _G_, back into the reservoir, _C_.

When all the air has been expelled from the apparatus and the pores of the bones, the cock on the pipe, _E_, is closed. The cylinder, _A_, is then heated so that a pressure of a few atmospheres prevails in it. The vapors now act energetically upon the bones, the dissolved fat collecting upon the cylinder; the cock on the pipe, _H_, is then opened, and the superheated fluid discharged under high pressure into the distilling apparatus, _J_, and the solvent is distilled off from the fat by means of steam. The vapors of the solvent pass through the pipe, _K_, into the condenser, _F_, and from there back into the reservoir, _C_.

When the manometer on _A_ indicates no pressure, the cock on the pipe, _H_, is closed and the cylinder, _A_, again heated, the pipe, _E_, being open, so that any solvent still adhering to the bones may escape to the condenser, _F_.

Figs. 37 and 38 illustrate Seltsam’s apparatus as improved by Th. Richter, whereby the operation becomes entirely free from danger, the vaporization of the solvent being effected by steam only, and the work is carried on continuously.

There are two extracting vessels, _A_ and _B_, of thick boiler-plate, and provided with false bottoms, _G_, upon which the bones are placed, steam being admitted into the space between the true and false bottoms. The extracting vessels are surrounded by the jackets, _C_, and are further provided with the vacuum gauges, _E_, and the air-cocks, _F_.

There are, in addition, two other vessels, _H_ and _J_, which contain water, a vessel, _K_, for the solvent, and an air-pump, _L_. The operation is carried on as follows:

The extracting vessels, _A_ and _B_, are charged with bones, all the cocks, with the exception of _M_ and _N_ closed, and the air-pump, _L_, is set in motion, whereby a vacuum is formed in _A_. When this is sufficiently large, water is admitted from _H_ through the cock _O_ into the space _P_. The water-cock is then closed and the steam-cock, _Q_, opened. The steam entering the space, _R_, brings the water in _P_ to the boiling-point, and the air-pump sucks off the steam formed after the cock, _N_, is opened. The air-pump is then stopped and all the cocks closed, except _S_. The solvent now passes from the vessel, _K_, into the space _P_, and after closing the cock _S_, is evaporated by the admission of steam into _R_. The water-cock is then opened and cold water admitted into the jacket _C_, the solvent saturated with fat being thereby condensed in _P_. The water is then discharged from _C_ and steam introduced into _R_{1}_, whereby the solvent is evaporated and forced into the extracting vessel _B_ by means of the air-pump, _L_, after closing the cocks _M_ and _V_.

The process is then repeated in _B_, after a vacuum has been created in the same manner as in _A_.

In the meanwhile the vacuum in _A_ is interrupted by opening the air-cock _F_ and the fat drawn off through _P_ by opening the cock _U_.

The bones freed from fat are removed through the manhole _D_, and _A_ is charged with fresh material while the operation is carried on in _B_. Thus the operation is continuous, the solvent passing without any loss whatever from one extracting vessel to the other.

Alfred Leuner’s apparatus. Fig. 39, works on the Soxhlet principle, without pressure, using solvent and steam simultaneously. The bones are placed in _A_ above the perforated false bottom _B_. _D_ is a steampipe by means of which the bones are steamed as a preliminary, the surplus steam escaping through the outlet pipe _E_. After steaming, water and benzine are run in from the reservoir _F_, into the space under the false bottom, and heated by the steam coil _P_. The vapors evolved are condensed in the worm _K_, and at first run back over the bones through the cock _L_, the vapor passing upwards to the worm through _R_, and the condensed liquid being divided into separate streams by the spreading plate _O_. After some time the cock _G_ is opened so that the condensed liquid runs into the reservoir _F_, instead of flowing back into _A_. When all the solvent has volatilized nothing but water condenses in the worm, which is known by means of a sampling cock attached to _A_, the draw-off cock _E_ is then opened and the watery gelatinous solution and oily matter run off into a suitable separating receptacle. The bones in _A_ are then discharged through a manhole, and _A_ being refilled, the whole operation is repeated.

_Extraction with hydrochloric acid._ If the bones are to be chiefly worked for glue, extraction with hydrochloric acid, which has been referred to in Chapter III, under “Bones and Cartilages” may be highly recommended, the bones being thereby freed from their mineral constituents and the glue-yielding substance remaining behind in a pure state. The bones are allowed to remain in contact with the acid till they are flexible and translucent. This may be readily recognized by laying upon the material in the vat a bone split in two. When the latter by the treatment with acid shows the characteristic appearance of swollen cartilage, _i. e._, has become translucent, extraction may be considered complete.

The solution is then drawn off through a tap immediately above the bottom of the vat into stoneware vessels, and conveyed to the evaporating pans. The tap is then closed and enough water to cover the cartilage is admitted into the vat, and the whole allowed to stand for a few hours in order to extract as much as possible the solution of bone-salts remaining in the cartilage. The fluid is then drawn off. It is a quite concentrated solution of bone-salts and, mixed with an equal volume of hydrochloric acid, may be used for the extraction of fresh quantities of bones, or be mixed with the fluid first drawn off and evaporated.

The further washing of the cartilage is effected by repeatedly pouring water over it, the operation being continued until the water running off shows no acid reaction. Washing has to be done very carefully, since glue-solution obtained from cartilage containing but a very small quantity of acid will not congeal. It is, therefore, advisable to add to the last wash-water 1 per cent. of soda, this quantity being quite sufficient for the neutralization of the last traces of acid.

_Sulphurous acid process._ In this country sulphurous acid is largely employed in the manufacture of glue derived from bones. When ordinary bones are treated with a current of moist sulphurous acid gas, they absorb from 10 to 12 per cent. of their weight of the gas in the course of 12 hours. The amount may increase to 15 or 20 per cent. on longer treatment, but the excess will then disappear on exposure to air. Messrs. Grillo and Schroeder of Düsseldorf, who patented this process in 1894, believe that this is simply due to the calcium phosphate present in the bones, and remark that an absorption of 11 to 12 per cent. on the gross weight amounts to 16 or 17 per cent. of the inorganic constituents, and corresponds to the equation:

Ca_{3}(PO_{4})_{2} + SO_{2} + H_{2}O = 2CaHPO_{4} + CaSO_{3},

the sulphurous acid simply acting in the same way as sulphuric acid does in the manufacture of superphosphate, but being a milder acid than sulphuric, the alteration of the organic constituents which are available for glue-stock can be almost entirely avoided. The acid phosphate is soluble in water, therefore the bones after treatment are easily disintegrated by boiling water when a large portion of the lime remains in the sediment, while the gelatine is dissolved.

The process as commercially conducted is very similar to the well-known sulphite method of treating paper pulp, and is carried on in iron cylinders or better in close wooden vats lined with lead.

The gas is usually generated in an impure form, with a large admixture of air and carbonic acid, by combustion of pyrites and coal, of crude sulphur, or even of only highly pyritous fuel.

On the other hand, since it is well established that the absorption of a diluted gas is less ready, and is more wasteful than that of a gas in a pure state, the employment of a definite quantity of sulphur dioxide in a concentrated state, either prepared by the regulated burning of sulphur, or the decomposition of sulphuric acid, yields more regular results, and a product of better quality. Liquid sulphur dioxide, which is now obtainable at a moderate price and in quantity, has the advantage that it yields a continuous current of pure gas of any required rapidity by simply opening a valve, and that the exact amount used can be ascertained by taring the containing vessel before and after the operation (S. Rideall).

The washed bones are brought into the above-mentioned cylinder or vat and treated with a saturated solution of sulphurous acid. The duration of the action of the acid varies according to the condition of the material and can only be determined by experience. The result of the process is a liquor almost as clear as water, which, after evaporation in the vacuum pan, is equal as regards clearness and lustre, to the best quality of glue prepared from waste of hide and skin. The fat extracted from the bleached bones is lighter in color and has not the disagreeable odor of ordinary bone fat, and consequently brings a better price.

For the generation of sulphurous acid Dr. Bruno Terne, of Mass., has constructed a very simple apparatus shown in Fig. 40. The sulphur is burned in _S_; _A_ is the escape pipe of stone; _T_, the collecting reservoir; _P_, the steam-pump for acid; _R_, chimney for the sulphur burner.

4. CONVERSION OF CARTILAGE INTO GLUE.

The conversion into glue of the swollen cartilage obtained by treatment with hydrochloric or sulphurous acid may be effected by continued boiling in open pans or in an apparatus recommended by Wm. Friedberg, and shown in Fig. 41.

The boiler _K_ of thick boiler-plate has a diameter equal to its height. Underneath the perforated false bottom _S_, which serves for the support of the bones, lies a perforated steam coil _R—D_ for the introduction of steam. To this steam coil is fitted a branch-pipe _d_, which reaches into the upper portion of the boiler into which also enters the water-pipe _W_. The apparatus is further fitted with a water-gauge, an air cock, sampling cock and manhole for the introduction of the cartilage.

The mode of operation with this apparatus is as follows: The boiler is filled three-quarters full with cartilage. Enough water to fill the boiler one-quarter full is then admitted through the pipe _W_, and the steam-cock _D_ opened. The steam passing out through the numerous perforations in the coil _R_, is at first condensed in the water, but soon brings the latter to the boiling-point, and from this stage on begins the formation of glue. The glue dissolves in the water, and a sample of the solution is from time to time drawn off through the sampling cock and tested as to its concentration. When the solution possesses the required concentration, the admission of steam through _R_ is interrupted, and the cock _d_ of the branch-pipe opened, as well as the cock of the discharge pipe _H_, the latter being opened gradually. The discharge pipe _H_ is connected with the perforated plate _F_, which is covered with a closely-woven cloth and thus acts as a filter, retaining all the solid particles suspended in the glue solution.

By opening the cock _d_ of the branch-pipe, the steam-pressure acts only upon the surface of the fluid, the latter being consequently pressed with great force through the filter-cloth.

When the hissing noise caused by escaping steam indicates that all the fluid has been removed from the boiler, the cock _d_ of the branch-pipe is closed, and through a rose fitted above the boiler, water is allowed to flow upon the latter. By this cooling the greater portion of the steam in the boiler is condensed and water may be admitted through _W_.

The operation of glue-boiling is then commenced anew by admitting steam into the steam coil, and continued until the cartilaginous mass has been reduced to about one-third of its original bulk, when the apparatus is opened, fresh material introduced, and the whole operation repeated.

In order to be able to replace the filter without having to empty the entire apparatus, it has been given the shape shown in Fig. 42. The upper portion of the discharge-pipe _A_ is connected with the lower portion by the box-screw _H_. In the latter is inserted a short cylinder, _C_, with a perforated bottom upon which is placed the filter-cloth; the latter is kept in position by the ring _R_.

For every apparatus two of the above-described filters will be required. If, notwithstanding a full steam pressure, the glue solution runs off sluggishly, it is indicative of the pores of the filter being choked up. The screw-box _H_ is then removed, the filter taken out and replaced by another one.

The glue solution discharged from the apparatus is in most cases sufficiently clear to allow of its being immediately evaporated. However, for the production of a particularly fine quality of glue, it is advisable to clarify the solution by settling. As the liquor has to be kept warm to allow of the solid particles to settle, W. Friedberg recommends the use of the apparatus shown in Fig. 43. It consists of an iron cylinder with a diameter equal to one-third of its height. The front of the cylinder is furnished with a number of cocks placed at equal distances from each other, and also with a pipe in the slightly conical bottom. It is surrounded by a wooden jacket, the intermediate space being filled with a bad conductor of heat. By this arrangement the liquor is kept warm and in a liquid state for several hours, giving ample time for the solid bodies held in suspension to settle on the bottom. The condition of the liquor is from time to time tested by allowing a small quantity of it to run into a glass from the lowest cock in front. If the sample is perfectly clear, the liquor may be drawn off. If, however, after several hours’ standing only the upper portions of the liquor are clear, while the lower ones are still turbid, further clarification by this means is impossible. The upper portions of the liquor are then used for finer qualities of glue and the lower ones for inferior grades.

By treating the cartilage with high-pressure steam, a liquor is obtained which on cooling congeals to quite a solid jelly, and it might be immediately brought into the forming-boxes, cut into cakes, and dried. However, as the drying of the glue is one of the most difficult operations for the glue-maker, it is of great advantage to obtain the liquor in as high a state of concentration as possible in order to obtain a solid jelly, which causes the least difficulty in drying. For this purpose the liquors leaving the clarifying vat with a strength of about 20 per cent. dry glue are evaporated down to a strength of about 32 per cent. in winter, and 35 per cent. in summer. Evaporation may be effected in open pans or in vacuum.

Fig. 44 shows the arrangement of an open evaporating pan. The copper pan _P_ has the form of a shallow cylinder with a slightly conical bottom, in the lowest point of which is the discharge pipe for the concentrated liquor. During the operation the discharge pipe is closed by the ball-valve _V_, which can be raised by the lever contrivance _M_. The pan is surrounded by an iron steam-jacket; the steam passes in at _D_, and the condensed water runs off at _A_. _H_ is a sampling cock for taking samples to test the concentration of the liquor.

To prevent the workroom from being filled with steam arising from the pan, the latter is covered with a hood of wood which terminates in the pipe _S_ projecting above the roof, and a narrow pipe _R_ branching off from the steam pipe _D_ passes into _S_.

When vapors commence to arise from the liquor, the cock on the pipe _R_ is slightly opened whereby a jet of steam is blown into the pipe _S_, the latter then acting as an exhauster, and the vapors in the hood _C_ are carried along by the jet of steam. By this arrangement no vapor passes into the workroom and steam is also very rapidly evolved from the surface of the liquor.

Sufficient steam should be admitted to the pan for the liquor to give out an abundance of vapor without, however, being brought to the boiling-point, as in that case foam would be formed and the liquor in cooling yield a product full of blisters. When the liquor has acquired the proper degree of concentration, the admission of steam to _D_ and _R_ is interrupted and the valve _V_ having been raised the liquor is run into the cooling-boxes. The latter are of wood lined with zinc, or better of stout zinc or heavily galvanized iron. They hold about ½ cwt. and are of two shapes: one deep and nearly square, another long and shallow, for quick cooling of clear liquors. Iron should not be used, as it readily rusts and causes discoloration of the glue.

Cooling is effected by cold water where it is available, but often merely by cold air, aided by fans or blowers, in a room protected from heat or frost. According to S. Rideal, refrigerating machines are now also employed, which, by the evaporation of liquid gases, such as ammonia, sulphurous or carbonic acid, reduce a tank of brine to near freezing-point. The temperature should not be allowed below 33° or 34° F., for if frozen the jelly is hard and difficult to cut. The brine circulates in iron pipes placed near the ceiling of the room; they must be kept as clear as possible of ice and dirt, and the cooling house should be scrupulously clean and sweet.

Spiral evaporators are recommended by Thomas Lambert as forming a ready and economical means of evaporation. The evaporator consists of a spiral steam coil, made of copper, and 2 inches in diameter, revolving on a centre shaft; the lower half of the coil is covered with the glue-liquor in the trough. The shaft rests on two plummer-blocks, one receiving the steam, and the other discharging the spent steam and condensed water. The shaft is hollow to the first coil, and the steam is thus conveyed to the spiral. From the last coil to the end of the plummer-block the shaft is also hollow, and in that portion resting on the block two openings are made. In the inside of the plummer-block, two openings are bored to the outside, each forming a covered channel; as the shaft revolves, all the holes directly face each other at intervals, and thus allow any condensed water in the coils to be blown through. From 25 to 28 coils are generally used in each spiral. The glue-liquors are fed into the trough at one end, and have a temperature of 75° F.; the temperature of the evaporated liquor is 85° F. In the rather slow passage through the trough, the liquors receiving the heat of the revolving coils are raised in strength from 20 per cent. to 32 per cent. dry glue, at which point they are ready for jellying.

_Vacuum pans_ are much used in this country for evaporating glue-liquors, though there are some complaints made of the great waste owing to spray and froth being carried off in the steam. As is well known the boiling-point is lowered by increasing the pressure on the surface of a fluid. By enclosing water in a vessel connected with a constantly working air-pump, it is brought to the boiling-point by heating to between 95° and 104° F. The construction of vacuum pans is based upon this principle, and such apparatus is largely used in many manufacturing processes for evaporating to a certain degree fluids, for instance, sugar solutions, which readily become decomposed at a higher temperature. Vacuum pans are also very suitable for the evaporation of glue-solution, especially in plants working on a large scale.

Fig. 45 represents an elevation of a vacuum pan for evaporating glue and gelatine liquors as described by Thomas Lambert. The pan is built of steel plates, and lined outside with wood work, and rests on a floor constructed of rolled steel plates, supported on four columns, with a stairway leading to the working platform. One half of the lower part is shown in section, giving a view of the coils by which the pan is heated. The various parts are as follows: _A_, the body of the pan; _B_, the dome; _C_, exhaust pipe leading from the dome to the condenser; _D_, condenser; _E_, air or vacuum pump; _F_, storage tank for glue or gelatine liquors, warmed with steam coil; _G_, supply pipe leading from storage tank to vacuum pan; _H_, discharge valve; _I_, barometer gauge for indicating vacuum; _J_, inlet steam pipe for supplying the coils; _K_, exhaust end of vacuum coils; _L_, iron staircase; _M_, steel floor.

The accessories to the pan are placed in a convenient position above the working floor and include a steam gauge for noting the pressure in the coils, a gauge for indicating the height of the liquor in the pan, vacuum gauge _I_, as shown in the drawing, air-cocks and a thermometer. The pan is also fitted with a small apparatus, by which portions of the boiling liquor can from time to time be drawn, without disarranging the vacuum, so that the progress of evaporation can be ascertained.

In working the pan, the storage tank _F_ is first filled with the weak glue liquors to be evaporated; the valve on the supply pipe _G_ is then closed, and the vacuum pump set in motion; a few strokes are sufficient to reduce the internal pressure, and the valve of the supply pipe is then opened, and the liquor allowed to fill up the pan to the desired mark on the gauge. The valve is then closed, the steam-inlet valve _J_, supplying the coils, opened. As the heat from the coils spreads through the liquor, the vacuum pump is kept steadily at work reducing the inside pressure to within 2 to 2½ inches of a perfect vacuum, as seen on the barometrical scale. In this vacuum the liquor will boil at 120° to 130° F., and the boiling is continued until the withdrawn samples, as tested by the glue-meter, show the desired strength. The pump is then stopped, the vacuum broken by opening the air-cocks, and the concentrated liquor is run through the valve _H_ into suitably arranged receiving tanks, for supplying the trays or glasses for jellying.

For economical working with large quantities of weak liquors, a combination of two, three and even four vacuum pans, forming the double, triple and quadruple effect evaporators, have been designed for concentration purposes. The triple effect is, however, the system mostly in use, and consists of a grouping of three cylindrical pans, each connected by suitably arranged piping, by which the vapors of the first pan are conveyed to and made to heat the coils of a second pan, the resulting vapors from the second, passing on to the third pan, for a similar purpose. All the pans are connected with powerful pumps, producing a nearly absolute vacuum in each. The liquor is evaporated to a given density in the first pan, and then passed on to the second, and ultimately to the third, at which stage 80 per cent. of its water will have been driven off.

To obtain in all cases a product of equal concentration, it is advisable to have an instrument which will indicate the amount of dry glue in the solution. (Fig. 46.)

By immersing a glass aerometer in the glue-liquor, the percentage of glue is indicated by a scale registering from 0 to 70 per cent. with the jelly or glue solution at a temperature of 167° F.

To measure the temperature quickly, a thermometer is added, and for the execution of the entire test, a sheet-iron vessel consisting of a large and two small tubes, _a_, which when not in use, serve for the reception of the glass instruments contained in a special case. For testing, the small cylinder is placed in the large tube, _a_, and filled with jelly by means of the cap which serves as a cover. The large tube is filled with hot water to bring the jelly to the required temperature. The two instruments are then immersed in the tubes filled with glue-liquors to be tested, and temperature as well as percentage can be readily read off.

The evaporated and cooled glue-liquor is cut into cakes and dried in the same manner as previously described.

5. PROCESS FOR THE SIMULTANEOUS UTILIZATION OF BONES FOR FAT, BONE-MEAL AND GLUE.

Manufacturers frequently sort the bones in such a way that materials of different quality are obtained. Thick, compact bones are utilized for the manufacture of animal charcoal, a comparatively small percentage of bone-meal resulting in crushing such bones.

Incompact, porous bones, on the other hand, yield not only crummy animal charcoal of less value, but in stamping also a larger percentage of bone-meal than compact bones. Hence they are as a rule directly worked for fat, glue and steamed bone-meal, no attempt being made to convert them into such granular pieces as are suitable for the production of charcoal.

For this purpose, the bones are first broken by a crusher or mill into coarse pieces, and the fat extracted by a special process or together with the glue in one operation. The latter method would seem to be the most suitable, time and labor being thereby saved, but it must be borne in mind that fat extracted by itself brings a much better price than that obtained by steaming, and besides the yield of glue is larger from steamed bones which have been previously degreased.

The crushed bones—whether degreased or not—are subjected to the action of high-pressure steam. The apparatus, Fig. 47, used for this purpose consists of a cylinder of thick boiler-plate, 10 to 13 feet high and 3 to 4 feet in diameter. _E_ and _A_ are manholes, which can be closed steam-tight. The pipe _D_ leads to the steam-boiler and opposite to _D_ is a short pipe, _H_. The cylinder is further fitted with the perforated false bottom, _S_, and the bent pipe, _L_.

As a rule, four to six, and in larger plants even more, of such cylinders are combined to a battery. In this case the discharge pipe, _L_, terminates in a common collecting vessel, and the steam-pipes, _D_, branch off from a main steampipe. The battery may be enclosed by brickwork, but is preferably placed upon a suitable foundation and surrounded by woodwork, the intermediate space between woodwork and cylinders being filled with sawdust. This plan offers the best means of keeping the heat together, and the further advantage that, in case one of the cylinders becomes defective, it can be readily taken out and replaced by a new one.

In order to be able to fill the cylinders rapidly and with the least expenditure of power, it is advisable to place the bone-crusher at such a height that the crushed bones fall directly into carriages which are run upon a small railway over the charging holes of the cylinders, and emptied. In front of the manholes, _A_, for discharging the bones is also a railway, so that the crushed bones can be directly emptied into carriages and conveyed to the stamping mill.

The cylinder having been filled with bones, is closed steam-tight. The cock, _H_, is then opened and steam admitted by opening the cock _D_. The steam passing in at first, is cooled off by coming in contact with the bones and condensed to water. However, the temperature in the cylinder soon becomes so high that the steam is no longer condensed and, having first expelled the air in the cylinder through the pipe _H_, it escapes through the latter in the form of a powerful jet. When this is the case _H_ is closed and high-pressure steam allowed to act upon the bones.

The fat contained in the bones melts and trickles down. On the bottom of the cylinder collects a fluid which contains glue, is of a milky turbidity due to admixed drops of fat, and with a quite thick layer of fat upon its surface. From time to time—about every hour—the cock _L_ is slightly opened. By the pressure of the steam the glue-liquor is expelled with great force through the pipe _L_, the latter being closed when by the peculiar noise it is noticed that only steam escapes.

Steaming and the occasional discharge of melted fat are continued until on testing a sample of the liquor running off, it is noticed to be free from fat. The liquor in the cylinder is then expelled by the pressure of steam, the steam-cock _D_ closed, the manhole _A_ opened, and steam again admitted through _D_. By the steam-pressure the greater portion of the bones in the cylinder is expelled through the manhole _A_. The bones coming from the cylinder are pliable and soft, and, after drying, are readily converted by grinding into bone-meal.

For the manufacture of animal charcoal it is of the utmost importance that steaming should be interrupted at the time when the bones are completely degreased. If, however, only fat, glue and bone-meal are to be produced, steaming may advantageously be continued for a longer time.

The longer the bones are subjected to the action of high-pressure steam, the more complete the conversion of glue-yielding substance into glue will be. To be sure, the bone-meal obtained from such bones will contain somewhat less nitrogen than the product from bones not steamed quite so long. However, the content of phosphates will in both cases be the same, and on this depends, in the main, the fertilizing value of bone-meal.

The fluid discharged from the cylinder consists of a mixture of glue-liquor and drops of fat. It is run into a large vat, in which it is kept warm for a few hours, when the fat rises and collects in a coherent mass on the surface. The fat is then drawn off through cocks in the upper portion of the vat, while the glue-liquor is discharged from the bottom of the vat, running first upon a very fine meshed sieve, which retains the coarser bodies held in suspension, and then directly into the evaporator. In the latter the liquor is evaporated to the desired strength, when it is run into the clarifying vats, and finally into the cooling vessels.

With the above-described process, the simultaneous utilization of the bones for animal charcoal is only possible if the crushed steamed bones are passed through a sieve for the purpose of sorting out the granular pieces of suitable size. However, in the process above described, incompact bones are, as a rule, used which give but a small percentage of granulated pieces, and the latter yield an inferior quality of animal charcoal. It is therefore best to use the steamed bones from which the fat and glue have been extracted for the production of bone-meal.

For the manufacture of animal charcoal, the bones have to be carefully sorted, fresh bones rich in organic substance being best for the purpose, and the hardest and thickest pieces should be selected. Previous to carbonization, the bones are degreased by extraction with benzine or carbon disulphide, and then crushed.

Carbonization was formerly effected in iron pots having a capacity of about 25 quarts each. However, by this process a uniform product of good quality cannot be obtained, and, besides, the total quantity of organic substance of the bones is lost. At present carbonization is effected in retorts, whereby large quantities of animal charcoal are in a comparatively short time obtained, and, besides, the products of destructive distillation can be completely utilized. An essential product of distillation is a large quantity of inflammable gases, which can be used for heating the retort-furnace or for illuminating the entire plant, it being, however, best to arrange the conduits so that the gases can be used for either purpose.

A detailed discussion of the methods for gaining and further working of the products of distillation is not within the scope of this work, and only a brief description of a plant for the manufacture of animal charcoal will here be given.

Figs. 48 and 49 show the arrangement of a Belgian retort-furnace, Fig. 48 representing a vertical section lengthways, and Fig. 49 a horizontal section. The illustrations, however, are given at different heights in order to show plainly the arrangement of the fire-place and the passage of the fire-gases.

The cast-iron retorts—sixteen in the apparatus shown—are placed in rows alongside and one after another, so as to be swept as uniformly as possible by the fire. As will be seen from Fig. 49 the firing is so arranged that only the upper portions of the retorts are touched by the flames. _B_ is the actual fire-place, and _A_ the ash-pit, both being furnished with closely fitting doors so that the fire may be properly regulated, and the retorts eventually be exclusively heated with gas.

The retorts are cylindrical in form, with one end closed. At the open end is fixed the frame or mouth-piece, which carries the door swung on a hinge. The door has a slight projecting rim, some two inches wide, which, with the surface of the frame, is ground perfectly true; on closing, the joint is made gas-tight by a lever arrangement.

The fire-gases escaping from _B_ are distributed as uniformly as possible by the flues, _a_, carried underneath the pans, _E_, and finally pass out in the direction of the arrows through a chimney.

At the time when the extraction of fat was exclusively effected by boiling the bones, the pans _E_ served for this purpose, and the spaces _D_, _D__{1}, _D__{2}, etc., alongside the pans, which were also heated by the fire-gases, were used for drying the bones. However, at present, the extraction of fat is, as a rule, effected by means of benzine or carbon disulphide, and it is advisable to replace the pans, _E_, by a bonekiln, and eventually to utilize any waste heat for heating the evaporators for glue-liquor.

Fixed to the upper portion of each retort is a pipe, and these pipes lead into a very wide iron-pipe, _T_. The products of destructive distillation escaping from the retorts combine in _T_, and besides having a very large diameter, this pipe must be considerably inclined to avoid the accumulation of products of distillation in it. To prevent the products of distillation from depositing in a crystalline form in _T_, the latter is covered with a bad conductor of heat.

The pipe _T_ is connected with a series of condensing vessels, _D_, another series of vessels being placed alongside the first one, so that the vapors may be conducted, as desired, into either one of them. Two batteries of such condensing vessels are required, as one of them has from time to time to be disengaged in order to be cleansed.

If the products of distillation would have to overcome the entire pressure of the column of fluid in the condensing vessels, their escape from the retorts would be very much retarded. To avoid this, plates are arranged horizontally a few inches below the level of the fluid, and the pipes dip into the condensing vessels only far enough to permit the escaping vapors to pass under these plates. By this arrangement, the vapors sweep under the plates and are absorbed by the fluids, a strong pressure in the apparatus being thus avoided.

The condensing batteries may of course consist of any number of vessels, but as a rule only a sufficient number to retain all the ammonia is employed, five being in most cases sufficient for this purpose. The last condenser is connected with an exhaust-pump, _p p_, which is kept in motion by a motor, _P_.

The pump removes all the bodies remaining in the last condenser and forces them, according to the position of the cock back of the pump, either into a glass-bell or through the pipe _H_ and the nozzles _a_ into the fire-place where they are burned.

To obtain the various products of distillation, the condensing vessels have to be filled with acid, and should therefore be constructed of lead or at least of sheet-iron lined with lead. The products to be obtained depend on the fluid used for filling the vessels; if filled with dilute sulphuric acid, ammonium sulphate is obtained, which may be utilized in the preparation of fertilizers. If hydrochloric acid is employed for the absorption of ammonia, solution of ammonium chloride is obtained, which may be crystallized by evaporation.

The products evolved in the destructive distillation of bones consist of various hydrocarbons and appear either as badly-smelling brown liquors—bone-tar—or as illuminating gas. The vapors contain further considerable quantities of ammonium carbonate and cyanide of ammonium. To obtain the latter, the last condensing vessel is filled with green vitriol solution, the cyanogen compounds remaining behind in the solution. If the gas escaping from the last condenser is to be used for illuminating purposes, it is freed from the greater portion of carbonic acid contained in it by purification with lime.

The mode of operation with Belgian retort furnaces is as follows: The retorts having been filled with comminuted degreased bones, the doors are closed perfectly gas-tight and firing is commenced. The pump is set in motion until a jet of gas blows through the nozzles. When this gas-jet burns with a luminous flame, destructive distillation of the bones is in full blast. The pump is then run with such velocity that the pressure in the interior of the retorts, as indicated by the manometer, is slightly greater than the external air-pressure, and the operation is thus continued so long as inflammable gases escape from the pipe _H_. The pump is then stopped, and one-half of the charcoal contained in the retorts is withdrawn to the canisters placed ready to receive it. The lids of the canisters are then luted down with a paste of char-dust and water, making an air-tight joint, and the charcoal is allowed to cool.

The retorts having been partially emptied, are at once refilled to the brim with crushed bones and closed gas-tight. There is but little or no loss of heat between withdrawing and charging, and distillation of the freshly-introduced bones commences immediately after charging, and is finished in a much shorter time than in the beginning of the operation.

In making animal charcoal on a large scale there are obtained from 2000 lbs. of raw material:

Animal charcoal, 1180 to 1220 lbs. Ammoniacal liquor, 178 to 180 lbs. Gas, 222 to 248 cubic yards.

However, these figures refer only to bones degreased by steaming, whereby a considerable portion of the cartilaginous substance passes into solution in the form of glue. In working bones degreased with benzine, larger yields than those indicated above are as a rule obtained. The ammoniacal liquor contains on an average 10 per cent. of ammonia. The gas freed from carbonic acid yields 2.7 times more light than good coal-gas.

6. PROCESS FOR THE SIMULTANEOUS UTILIZATION OF THE BONES FOR FAT, GLUE AND CALCIUM PHOSPHATE.

This process differs from the one previously described in that, in addition to the total quantity of fat and glue-yielding substance contained in the bones, the mineral salts are also obtained in a pure state, and can be further utilized.

The bones are degreased either by extraction with benzine or carbon disulphide or steaming, the operation in the latter case being continued so long as fat is yielded by the bones. The resulting glue-liquor is used in place of water for boiling the cartilage.

The bones are placed in large wooden vats furnished with well-fitting lids, and hydrochloric acid of 12 per cent. poured over them so that they are covered a few inches deep. With the use of acid of 1.04 specific gravity the greater portion of the salts contained in the bones will pass into solution in 48 to 72 hours, when the solution is drawn off as completely as possible from the vats.

The residue in the vats is treated with less concentrated hydrochloric acid and left in contact with it until the bones are soft and flexible and the thinner pieces have become translucent, this being a proof that all the mineral salts have been extracted, and nothing but pure cartilaginous substance remains behind. The solution is then drawn off, and after pouring repeatedly small quantities of pure water over the cartilage to expel the last remnants of acid liquor, it is subjected to thorough washing until the last traces of acid have been removed.

The resulting cartilage is white, translucent, and water-soaked. If left in this state it would of course soon putrefy, and it is best to work it at once, or if this cannot be done it will have to be treated with carbolic acid in the manner previously described, or dried.

Drying the cartilage is time-consuming work, and can properly be done only by artificial heat in kilns. If carefully protected from moisture, thoroughly dried cartilage may be kept without injury for any length of time. However, before being worked to glue, such material has to be again soaked in water previous to the actual boiling operation, and this process requires considerable time. It is therefore best to preserve it in carbolic acid solution, which only needs to be drawn off when the cartilage is to be worked, and may be further utilized.

If boiled in open vessels with water, 6 to 8 hours are required for the complete disintegration of the cartilage. In a closed apparatus under high pressure solution is effected in a much shorter time and the operation progresses very smoothly. With proper attention the glue obtained from bones degreased with benzine and freed from bone-earth by extraction with hydrochloric acid is, as a rule, very clear, and may be bleached with sulphurous acid.

The extraction of the phosphates from bones may be effected in a very suitable manner as follows: A number of vats filled with bones are placed in terraces one above the other, and the acid is allowed to run first into the uppermost vat. After having been for several hours in contact with the bones it is discharged into the next vat, fresh acid being run into the first one, and so on. By this process a highly concentrated solution of phosphates is in a few hours obtained in the lowest vat, solution still adhering to the bones in the other vats being finally expelled by water.

However, the process of extraction under decreased pressure is the most advantageous, it requiring least time. For this purpose the bones are brought into a vessel which can be closed air-tight and the air is expelled. When but a slight air-pressure prevails in the vessel, the cock of a reservoir filled with hydrochloric acid is opened, the external air-pressure now forcing hydrochloric acid into the extracting vessel.

Bones, as viewed under the microscope, consist of a mass permeated with numerous minute tubes or pores. When the air is expelled from the vessel containing the bones, the air in the pores of the latter is rarefied and the hollow spaces are filled with hydrochloric acid whereby solution of the phosphates is effected.

The yield of glue obtained from cartilage after extraction of the mineral constituents varies according to the compactness of the bones used. Solid and compact bones yield, on an average, 15 per cent. of dry glue, but a comparatively large quantity of calcium phosphate. On the other hand, porous bones rich in cartilage yield from 20 to 25 per cent. of dry glue. The liquor obtained by treating the bones contains, as previously mentioned, calcium phosphate, magnesium phosphate and calcium chloride in solution, and may be utilized in the manufacture of fertilizers or of phosphorus.

For the first purpose which is not very remunerative, but is not very troublesome, the liquor is treated with milk of lime until it is slightly alkaline, whereby a finely divided precipitate of basic calcium phosphate is obtained, whilst calcium chloride remains in solution. The precipitate is allowed to settle, separated from the supernatant fluid and dried. The resulting product contains, on an average, 65 per cent. calcium phosphate, up to 20 per cent. water and 10 to 15 per cent. calcium carbonate, quick lime and accidental impurities. It forms an excellent fertilizer.

If the liquor is to be utilized in the manufacture of phosphorus, it is evaporated in shallow pans of glazed stoneware. In cooling crystals of acid calcium phosphate are formed, which are separated from the mother-liquor. This subject will be more fully referred to in the next chapter.