Chapter 9

_The Freezer_.--This consists of a rectangular iron tank, 1 meter x 1 meter x 1.5 meters, containing a galvanized plate iron cylinder, A, kept in place by iron supports. This cylinder contains 24 horizontal tubes, which are open at the ends and riveted to vertical plates like those of tubular steam boilers. The tank is filled with a mixture of water and chloride of calcium, forming, as well known, an incongealable liquid. Into this liquid are plunged the receptacles containing the water to be converted into ice. The chloride of methyl is introduced through the cock, B, into the body of the cylinder, A, and surrounds and cools the tubes, as well as the incongealable liquid uninterruptedly circulating in the latter, by means of a helix, C, set in motion by a belt from the shop. This liquid is thus greatly lowered in temperature and freezes the water in the receptacles.

_The Pump_.--The pump in the larger apparatus has two chambers of unequal diameter, that is to say, it operates after the manner of compound engines.

The machine under consideration, being one that produces a moderate quantity of ice, has but a single chamber, as shown in Figs 4, 5, and 6. It is a suction and force pump, whose piston, E, is solid and formed of two parts, which are set into each other, and the flanges of which hold a series of bronze segments.

The chamber, properly so-called, is of iron, cast in one piece, and is surmounted with a rectangular tank, F, in which constantly circulates the cold water designed for cooling the sides of the cylinder; these latter always tending to become heated through the compression of the methyl chloride.

The cylinder heads are hollowed out in the middle, and carry the seats of the suction valves. Each of the latter communicates with a chamber, G G¹, in which debouches the pipe, H, communicating with the cylinder, A, of the freezer (Figs. 1, 2, and 3).

Above the cylinder there are two delivery valves which give access to the chamber, D, communicating with the worm of the liquefier (Fig. 7) through the pipe, J.

The piston of the pump is set in motion by a pulley, K, and a cranked shaft actuated by a belt from the shafting. The piston head is guided by a slide keyed to the frame.

_The Liquefier_.--This apparatus consists of a cylindrical tank, L, of 3 mm. thick boiler plate, mounted vertically on a masonry base and designed to be constantly fed with cool water. It contains a second cylindrical tank, M, of 6 mm. thick galvanized iron. This latter tank is provided with a cast-iron cover, on which are mounted the worm, N, and a pipe, O, connected with the tube of the pressure gauge. To the base of the tank, M, there is affixed, on a cast iron thimble, a cock, P, for setting up a communication between the tank and the pipe, R, which returns to the freezer through the cock, B (Fig. 1).

The cold water requisite for condensation enters the tank, L, through a pipe terminating in a pump or a reservoir. The waste water flows off through the tubulure, Q. The tank is emptied, when necessary, through the blow-off cock, S.

_Operation of the Apparatus_.--As has been remarked above, the cylinder, A, is filled with chloride of methyl. The pump, through suction, produces in this cylinder a depression from which there results the evaporation of a portion of the chloride of methyl, and consequently a depression of temperature which is transmitted to the incongealable liquid circulating in the tubes, and to the receptacles (carafes or otherwise) containing the water to be converted into ice.

The pump sucks in the vapor of mythyl chloride through the pipe, H, and through its suction valves, and forces it into the chamber, D, through its delivery valves, and from thence into the worm, N, through the pipe, J. Under the influence of compression and of the water contained in the tank, L, the methyl chloride liquefies and falls into the receptacle, M, from whence it returns to the freezer through the pipe, R.

Two pressure-gauges, one of them fixed on the freezer and the other on the liquefier, permit of regulating the running of the machine. The vacuum in the freezer is 0 to ½ atmosphere, and the pressure in the liquefier is 3 to 4 atmospheres. These apparatus make opaque ice, but will likewise produce transparent, if a pump for injecting air is adjoined. This, however, doubles the time that it takes to effect the freezing, and carries with it the necessity of doubling the number of moulds to have the same quantity of ice.

The cost price of ice made by this system depends evidently on the price of coal in each country, on the perfection of the boiler and motor, as well as on the power of the freezing machine. Putting the coal at 20 francs per ton, and the consumption at 2 kilogrammes per horse and per hour, ice may be obtained at a cost of about half a centime per kilogramme. The apparatus shown in the accompanying figures have been constructed according to the following data:

Production of ice per hour............ 25 kilogrammes. Production of heat units per hour..... 2.5 grammes. Quantity of ice produced per kilogramme of coal burned........... 5 kilogrammes. Water of condensation per hour........ 0.75 cubic meter.

These machines are employed not only for the manufacture of ice, but also in breweries for cooling the air of the cellars and fermenting rooms, or that of the vats themselves; in manufactories of chemical products; in distilleries; in manufactories of aerated waters, etc.

They may also be used in the carrying of meats and other food products across the ocean, and, in a word, in all industries in which it is necessary to obtain artificial cold.

The power necessary to operate apparatus that produce 25 kilogrammes per hour is about that of 3 horses.--_Annales Industrielles_.

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CARBONIC ACID IN THE AIR.

[Footnote: An address before the Paris Academy.]

By M. DUMAS.

Of all the gases that the atmosphere contains, there is one which offers a special interest, as well on account of the part ascribed to it in the mutual interchange going on between the two organic kingdoms, as on account of the relation that it has been observed to occupy between earth, air, and water; this gas is carbonic acid.

Ever since the fact has been established that animals consume oxygen and give out carbonic acid as the product of respiration, while plants consume carbonic acid and give out oxygen, the question has often been asked whether the quantity of carbonic acid contained in the air did not represent a sort of sustaining reservoir which was being continually drawn on by the plants and resupplied by animals, so that it has doubtless remained unchanged owing to this double action.

On the other hand, Boussingault has long since shown that volcanic regions give out through crevices and fumaroles enormous quantities of carbonic acid. The deposition of carbonate of lime that is continually taking place on the sea-bottom is, on the other hand, fixing carbonic acid in quantities which we may accurately estimate from the strata of limestone seen on the surface of the earth. We might imagine, that in comparison with the huge volumes of carbonic acid sent forth in volcanic districts, even in the oldest one, and the mass of carbonate of lime deposited on the sea bottom, the results attributed to the life of plants and animals would be of no consequence either for increasing or diminishing the physiological carbonic acid in the air comparable with those which are accomplished by the purely geological exchange.

Schloesing has recently succeeded, by a happy application of the principle of dissociation, in showing that the amount of carbonic acid in the air bears a direct relation to the quantity of bicarbonate of lime dissolved in sea water. If the quantity of carbonic acid diminishes, the bicarbonate of the water is decomposed, half of its carbonic acid escapes into the atmosphere, and the neutral carbonate of lime is precipitated. The aqueous vapor condensed from the air dissolves part of the carbonic acid contained therein, and carries it along, when it falls as rain upon the earth, and takes up there enough lime to form the bicarbonate, which is thus carried back to the sea.

The physiological role of carbonic acid, its geognostic influence, and its relations to most ordinary meteorological phenomena on the earth's surface--all these contribute to give special weight to studies concerned in the estimation of the normal quantity of carbonic acid in the air.

Nevertheless, this estimation is attended with great difficulty. Not everyone is able to take up such questions, and not all processes are adapted to it. The first thought which would naturally arise would be to inclose a known volume of air in a given vessel, and then determine its carbonic acid by measuring or weighing it. In this way we should obtain the exact relation between a volume of air and the volume of carbonic acid in it, for any given moment, and in any given place. If, however, this be done with a ten-liter flask, for example, it would only hold 3 c.c. of carbonic acid, weighing 6 milligrammes; and, whether it is weighed or measured, the error may easily equal 10 per cent. of the real value, hence no deductions could be drawn from the observed facts.

For this reason larger volumes of air were taken, and a current of air, whose volume could be accurately measured by known methods, was passed through condensers capable of retaining the carbonic acid. But in this case the air must pass very slowly through it, so that the process may last several hours; and since the air is continually in motion, owing to vertical and horizontal currents, the experiment may be begun with the air of one place, and concluded with air from a far distant spot. For example, if an experiment lasting twenty-four hours was made in Paris when the air moved but four meters per second (nine or ten miles per hour), it might be begun with air from the Department of the Seine, and end with air from the Department of the Rhone, or the Belgian frontier, according to the direction of the wind.

So long as we had no analytical methods of sufficient delicacy to estimate with certainty the hundredth, or at least the tenth of a milligramme of carbonic acid, it was very difficult to determine the quantity in the air at a given time and place. It is frequently possible to analyze upon the plain air that has descended from the heights above, and to examine by bright daylight the effect of night upon the atmosphere.

Still other difficulties show themselves in such investigations. It seems very easy to collect carbonic acid in potash tubes, and to determine its amount from the increase in weight of the tubes; but, alas! to how many sources of error is this method exposed. If the potash has been in contact with any organic substance, it will absorb oxygen. If the pumice that takes the place of the potash contains protoxide of iron, it will also absorb oxygen. In both cases the oxygen increases the weight of the carbonic acid.

Every experimenter who has been compelled to repeat the weighing of a somewhat complicate piece of apparatus, with an interval of several hours between, knows how many inaccuracies he is exposed to if he is compelled to take into calculation the changes of temperature and pressure, and the moisture on the surface of the apparatus. After fighting all these difficulties, and frequently in vain, the experimenter begins to mistrust every result that depends only on difference in weight, and to prefer those methods whereby the substance to be estimated can be isolated, so that it can be seen and handled, weighed or measured, in a free state, and in its own natural condition.

The classical experiments of Thenard, of Th. de Saussure, of Messrs. Boussingault, on the quantity of carbonic acid in the air, are well known to every one: they need only to be organized, repeated, and multiplied.

J. Reiset, who has conducted a long and tedious series of experiments on this subject, has adopted a process that seems to offer every guarantee of accuracy. The air that furnishes the carbonic acid is aspirated through the absorption apparatus by two aspirators of 600 liters capacity. The temperature and pressure of the air are carefully measured. The carbonic acid is absorbed by baryta water in three bulb apparatus. The last bulb, which serves as a check to control the operation, remains clear, and proves that no binoxide of barium is formed. The baryta water used is titrated before and after the operation, and from the difference is calculated the quantity of carbonate formed, and hence of the carbonic acid.

These tedious experiments, which varied in duration from 6 to 25 hours, require at least two days of continuous labor. They were repeated 193 times by Reiset in 1872, 1873, and 1879. They were made in still weather, and in violent winds and storms. The air was taken at the sea-shore, in the middle of the fields, on the level earth, during harvests, in the forests, and in Paris. Under such varied conditions, the quantity of carbonic acid varied but little; the numbers obtained were between 2.94 and 3.1, which may be taken as a general average of the carbonic acid in the air.

The quantity of carbonic acid in the free atmosphere is tolerably constant, which must necessarily be the case according to Schloesing's proposed relation between the bi-carbonate of lime in the sea and the carbonic acid in the air. The only cause that seems at all competent to change the geological quantity of carbonic acid in the atmosphere is the formation of fog. As the aqueous vapors condense, they collect the carbonic acid; and the foggy air, as a rule, is more heavily laden with this gas than ordinary air.

It is not surprising that there is less carbonic acid in the air collected on clear summer days, in the midst of clover, etc., that is in an active reducing furnace; if anything is surprising, it is that the quantity of carbonic acid does not sink below 2.8.

It is also a matter for surprise that in Paris, among so many sources of carbonic acid, the furnace fires, the respiration of men and animals, and the spontaneous decomposition and decay of organic substances, the quantity of carbonic acid does not exceed 3.5.

If, then, the great general mean of normal atmospheric carbonic acid deviates but little from 2.9 or 3.0, it is not doubtful that under local conditions, in closed places, and under exceptional meteorological conditions, considerable variations may occur in these proportions. But these variations do not affect the general laws of the composition of the atmosphere.

There are two entirely distinct points from which the measurement of the atmospheric carbonic acid may be contemplated.

The first consists in considering it as a geological element which belongs to the gaseous envelope of the earth in general, and it leads us to express the general relation of carbonic acid to the quantity of air, as about three volumes in 10,000.

The second, which relates to accidental and local phenomena, to the activity of man and beast, to the effect of fires and of decomposing organic matter, to volcanic emanations, and finally to the action of clouds and rain, permits us to recognize the changes which can occur in air exposed to the influences mentioned, and to a certain extent confined. Without denying that it is of interest from a meteorological and hygienic standpoint, it does not take the same rank as first.

J. Reiset's experiments, by their number, accuracy, the large volumes employed, and the interval of years that separate them, have definitely established two facts on which the earth's history must depend: the first is, that the percentage of carbonic acid in the air scarcely changes; the second, that it differs but little from three ten-thousandths by volume.

These results are fully confirmed by the results which were obtained by Franz Schulze, in Rostock, in 1868, 1869, 1870, and 1871. The averages which he got, with very small variation, were 2.8668 for 1869, 2.9052 for 1870, and 3.0126 for 1871.

More recently Muentz and Aubin have analyzed air collected on the plains near Paris, on the Pic du Midi, and on the top of Puy-de-Dome. Their results agree with those published by Reiset and Schulze.

The grand average of carbonic oxide in the air seems to be tolerably fixed, but after this starting-point is established it remains to study the variations that it is capable of, not from local causes, which are of little importance, but from general causes connected with large movements of the air. Upon this study, which demands the co-operation of a definite number of observers stationed at different and distant points of the earth, the experiments being made simultaneously and by comparable methods.

M. Dumas called the attention of the Academy to this point, in connection with its mission of selecting suitable stations for observing the transit of Venus. The process and apparatus of Muentz and Aubin offer the means adapted for making these experiments, and seem sufficient to solve the problem which science proposes, of determining the present quantity of carbonic acid in the air.

If these experiments yield satisfactory results, as we have good reasons to believe they will, it is to be hoped that annual observations will be made in properly-chosen places, so as to determine the variations which may possibly take place in the relative quantity of atmospheric carbonic acid during the coming century.--_Compt. Rend_., p. 589.

[Although this proposition was made by a Frenchman to his fellow scientists, would it not be well for some American to accept the challenge, and bring it before the coming meeting of the American Association for the Advancement of Science, in the hope that we, too, may contribute our mite of effort in the same direction?--_Ed. Knowledge_.]

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THE INFLUENCE OF AQUEOUS VAPOR ON THE EXPLOSION OF CARBONIC OXIDE AND OXYGEN.

[Footnote: Read before the British Association, Southampton Meeting, Section B, 1882.]

By HAROLD B. DIXON, M.A., Millard Lecturer in Chemistry, Balliol and Trinity Colleges, Oxford.

Two years ago I had the honor of showing before the Chemical Section of the British Association some experiments, in which a well-dried mixture of carbonic oxide and oxygen was submitted to electric sparks without exploding.[1] It was further shown that the introduction of a very minute quantity of aqueous vapor into the non-explosive mixture was sufficient to cause explosive combination between the gases when the spark was passed. The hypothesis advanced to account for the observed facts was that carbonic oxide does not unite directly with oxygen at a high temperature, but only indirectly through the intervention of water-vapor present, a molecule of water being decomposed by one of carbonic oxide to form a molecule of carbonic acid and one of free hydrogen, and the latter uniting with the oxygen to re-form a molecule of water, which again undergoes the same cycle of changes, till all the oxygen is transferred to the carbonic oxide:

H_{2}O + CO = H_{2} + CO_{2}

H_{2} + O = H_{2}O

[Footnote 1: "Report of British Association," 1880, p. 503.]

For such a series of reactions a _comparatively_ few molecules of water would suffice, and the change produced by their alternate reduction and oxidation would come under the old term of "catalytic action," inasmuch as the few water molecules present at the beginning are found in the same state at the completion of the reaction.

The truth of this hypothesis has since been confirmed by experiments I have made on the incomplete combustion of mixtures of carbonic oxide and hydrogen; and on the velocity of explosion of carbonic oxide and oxygen with varying proportions of aqueous vapor. I therefore thought a description of the more convenient methods lately devised as lecture experiments for showing the influence of water on the combustion of carbonic oxide would not be uninteresting to the Section.

A glass tube from 18 inches to 2 feet long, closed at one end, and provided with platinum wires, is bent near its open end so that the shorter arm makes an angle of about 60° with the longer arm. The tube, held by a clamp, is heated in a Bunsen flame, and is then filled with mercury heated to about 130° C. The mixture of gases is then made to displace a portion of the mercury by forcing it through a fine tube, which is connected by a steel cap to the eudiometer of McLeod's gas apparatus, and passes down through the mercury in the shorter arm of the experimental tube. When a sufficient quantity of the gaseous mixture has been collected in the longer arm, some dry phosphoric oxide is introduced in the following way: A small glass tube is heated, packed with the dry powder, and pushed down into the shorter arm of the experimental tube. With a hot glass rod the phosphoric oxide is pushed out at the bottom of the small tube, and passes up into the gaseous mixture in the longer arm. After standing for a few hours in contact with the phosphoric oxide, the gases may be submitted to strong sparks from a Leyden jar without igniting. Care must be taken that none of the oxide comes in contact with the platinum wires, for if any sticks to the wires it becomes heated by the passage of the sparks, and gives off enough water to determine the explosion. In this way I have prepared several specimens of a non-explosive mixture of carbonic oxide and oxygen in the proper proportions to form carbonic acid. Some of these tubes have been submitted without explosion to sparks from a large Leyden jar, to a continuous succession of sparks from a Holtz machine, and to the discharge of a Ruhmkorff's coil, that heated the platinum wires between which it passed to bright redness. Other tubes which withstood the passage of the sparks from a Leyden jar, when submitted to the discharge of the coil, exploded after a few seconds when the platinum wires became red-hot. This I think may probably be attributed to hydrogen, occluded by the platinum, being given off on heating, and forming steam with the oxygen present.

For an easy and striking lecture experiment, I employ a tube open at both ends and bent like a W. The two open arms are short and the platinum wires are fixed at the highest bend. The tube is filled with hot mercury--one of the ends being closed by a caoutchouc stopper for the purpose--and a dry mixture of 5 volumes of air and 2 volumes of carbonic oxide is introduced into the bent tube over the mercury. A little phosphoric oxide is passed up one arm. After a few minutes the gases may be submitted to the spark without exploding. A little water may then be introduced through a pipette into the other arm; and if the spark is passed directly the gases ignite in the wet and not in the dry arm of the tube.

The admixture of the inert nitrogen renders a larger quantity of aqueous vapor necessary for the explosion than when only carbonic oxide and oxygen in proper proportion are present.

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COMPOSITION OF BEERS MADE PARTLY FROM RAW GRAIN.