Part II., by Wood, Sand and Law, J.S.C.I. 1911, p. 872.

Puer liquors, titrated in this manner with N/10 soda or HCl until a potential of 0·69 volts is reached (at which point phenolphthalein turns from colourless to red), gave the following results per 100 c.c. filtered liquor.

---+-----------------+--------+--------------------------------- | | | {Alkali} No.| Description of |Reaction|c.c. N/10 { or } Required for | Liquor | | { Acid } Neutralization ---+-----------------+--------+--------------------------------- 1 |{New puer }| Acid | 7·4 alkali |{before goods }| | | | | 2 |{The same after }|Alkaline| 0·57 acid |{goods }| | | | | 3 |{New puer }| Acid | 8·1 alkali |{before goods }| | | | | 4 |{The same after }|Alkaline| 3·25 acid |{goods }| | | | | 5 |{Used puer after}|Alkaline| 5·00 acid |{goods }| | | | | 6 |Spent puer |Alkaline| 6·6 acid ---+-----------------+--------+---------------------------------

Determinations made in this way are more accurate than the colorimetric method above described. Puer liquors after use are found by the electrometric method generally on the alkaline side, whereas such liquors show neutral to phenolphthalein owing to some action of the bate constituents on the indicator.

PUER LIQUORS IN ELECTROMETRIC APPARATUS.

---+--------+---------+-------+---------+------------------------- | | c.c. of | | c.c. of | | P.D. |N/10 KOH | P.D. |N/10 HCl | | Before |required After |required | No.| Skins | for | Skins | for | Description of Liquor | | 50 c.c. | | 50 c.c. | | volt |liquor to| volt |liquor to| | |0·69 volt| |0·69 volt| ---+--------+---------+-------+---------+------------------------- A1 | 0·622 | 1·95 | .. | .. | New puer A2 | .. | .. | 0·747 | 1·9 | A1 after 2 packs skins B1 | ·607 | 3·7 | .. | .. | New puer B2 | .. | .. | ·770 | 3·1 | B1 after 3 packs skins C1 | ·580 | 4·3 | .. | .. | New puer C2 | .. | .. | ·762 | 2·8 | C1 after 2 packs skins D1 | ·580 | 5·2 | .. | .. | New puer D2 | .. | 0·8 | ·680 | .. |{D1 after 1 pack skins | | | | |{(still on the acid side | | | | |{ of 0·69) E1 | ·570 | 6·2 | .. | .. | D2 + 4 buckets more puer E2 | .. | .. | ·720 | 0·8 | E1 after 1 pack skins F1 | ·610 | 3·0 | .. | .. | New puer F2 | .. | 0·5 | ·680 | .. |{F1 after 1 pack skins | | | | |{(still on the acid side | | | | |{of 0·69) ---+--------+---------+-------+---------+-------------------------

The potential at the neutral point, using an auxiliary electrode filled with N/1 potassium chloride, was 0·69 volts. Potentials below this point, therefore, indicate an acid reaction, and, conversely, potentials above 0·69 volts indicate an alkaline reaction. The apparatus is thus very useful in following the course of the bating process. The initial potential also enables the hydrion concentration of the bate liquor to be determined directly (see Chapter III.). It was found that during the bating of a pack of skins the hydrion concentration of the liquor diminished from 10^{-5·2} to 10^{-7·4} normal.

Such acidities as are found by this method, e.g. 7–8 c.c. N/10 acid per 100 c.c., are too great if made up by taking a solution of the free acids. The ionization must be almost entirely repressed by the presence of sufficient neutral salts of the same acids in the bating liquid, in order to obtain a similar result to the puer.

The table opposite gives the values found for a series of puer liquors, made with dog manure from hunting-kennels.

The reaction of puer liquors (expressed in c.c. of N/1 alkali or acid per 1000 c.c. of bate), in a series in which the washed goods were run for one hour in old puer, out of which goods had been taken, and then transferred to a freshly-made puer, was as follows:--

----------+----------------+-------------- Bate | Before Goods | After Goods ----------+----------------+-------------- Old puer | Neutral | 28 c.c. acid Fresh | 10 c.c. alkali | Neutral Old puer | Neutral | 40 c.c. acid Fresh | 11 c.c. alkali | Neutral ----------+----------------+--------------

These figures were obtained by the ordinary colorimetric method described on p. 32.

Adding together the alkalinity of the bate run away, and the acidity of the new bate which is neutralized, the total for 1 and 2 = 38 c.c. N/1 acid, and for 3 and 4, 51 c.c. N/1 acid.

Multiplying by 810 (volume of bate less volume of dry skin), we find that the skins have neutralized 30·78 and 41·31 litres N/1 acid respectively. Since one litre of N/1 acid will neutralize 28 grm. of lime, it follows that the lime removed by the bate was in one case 862 grm., in the other case 1157 grm.

The 500 kilos, of wet washed grains introduced into the paddle contain approximately 0·5 per cent. of lime CaO, equivalent to 2500 grm. altogether. As will be seen from the figures given in table, p. 35, the free acids present in the fresh bate are only capable of neutralizing 10 × 810 c.c. of normal alkali, and this is equivalent to 225 grm. of CaO, or 9 per cent. of the total lime.[30]

[30] Eitner, Der Gerber, 566, p. 77, states that little or no neutralization of lime by acids or acid salts takes place in the bate.

In addition to this, a considerable amount of lime is removed from the skins which is not found afterwards in solution, but which is precipitated in an insoluble form during the course of the bating process. In an experiment to determine the amount of this insoluble lime, the following figures were obtained:--

LIME IN PUER LIQUOR, GRAMS PER LITRE CaO.

Before Skins After Skins Soluble lime 0·19 0·485 Insoluble lime 0·13 0·485 ---- ----- 0·32 0·970

It will be noted that the amount of insoluble lime has increased to a greater extent than the soluble lime, showing that part of the lime from the skins has been precipitated in an insoluble form. The increase of lime in the solution is 0·295 grm. per litre of soluble lime, and 0·355 grm. per litre of insoluble lime, a total increase of 0·650 grm.; that is, the ratio of soluble lime to insoluble lime in the original puer is 1·46 : 1. In the used puer it is 1 : 1, so that in the above case more than half the lime removed from the skins (54·5 per cent.) has been precipitated in an insoluble form, some of it in the form of calcium phosphate, and the remainder probably in the form of calcium oxalate. Crystals of calcium oxalate may be seen under the microscope in the puer residues.

The oxalic acid is produced by bacterial action, as has been shown by Zopf[31] and Banning[32]; but it is difficult to estimate it in the bate, and a reliable analytical method of doing so requires devising.

[31] Ber. Deutsch. Botan. Ges. 1900, xviii. 32, J.S.C.I. 1900, 386.

[32] Centr. Bakt. Parasitenk, ii. Abt. viii., 393, etc., J.S.C.I. 1902, 1151.

A further 30 per cent. to 40 per cent. of the lime in the skins is removed by the chemical action of the complex amines of the organic acids, and by the mechanical effect of the paddle or drum, so that the final result is more or less as follows:--

Per cent. Lime neutralized by free acids 9 Lime dissolved by complex amines 25 Lime precipitated 30 Lime remaining in the skins 36 --- 100

The lime remaining in the skins is not in the caustic state, but principally in the form of neutral salts (see footnote to p. 24).

Some of these neutral salts appear to be absorbed by the skin during the course of the bating, for, on examining pieces of the same skin at frequent intervals during the operation, and determining the ash, it was found that a minimum point for ash content was reached in about ten minutes, after which the ash actually increased. The results are shown in the following table, and also graphically by the curves, Fig. 7--in which A is a “grain” previously washed as free from lime as possible; B, an unwashed “roan.” The effect of the absorption of inorganic matter is very noticeable in both cases. A part of the effect only is apparent since a certain amount of skin substance passed into solution.

VARIATION OF ASH PERCENTAGE DURING PUERING.

-----------------+-------------------------------- -- | Ash, per cent. on Dried Skin -----------------+----------------+--------------- Time of Puering | Washed Delimed | Limed Roan (B) in Minutes | Grain (A) | -----------------+----------------+--------------- 0 | 2·88 | 7·03 5 | 2·91 | -- 10 | 3·20 | 3·54 15 | 4·80 | -- 20 | 4·08 | 4·24 25 | 4·29 | -- 30 | 4·85 | 5·19 35 | 4·59 | -- 40 | 4·70 | 5·45 45 | 4·60 | -- 50 | 4·45 | 5·67 55 | 4·42 | -- 60 | -- | 5·00 -----------------+----------------+---------------

The following analyses give the amount of lime in solution before and after the bating of the skins, in grams per litre of filtered puer liquor:--

---+-----------------+------------+-----------+--------------- No.| Bate |Before Goods|After Goods|Increase of CaO ---+-----------------+------------+-----------+--------------- 1 |Fresh puer | 0·108 | 0·62 | 0·512 2 | " " | 0·34 | 0·72 | 0·38 3 | " " | 0·20 | 0·52 | 0·32 4 | " " | 0·19 | 0·49 | 0·30 5 |Old puer | 0·54 | 0·84 | 0·30 6 |{Old puer, goods}| 0·98 | 1·38 | 0·40 |{not washed. }| | | 7 |{French puer }| 0·308 | 0·548 | 0·24 |{shop. }| | | ---+-----------------+------------+-----------+---------------

The limit of the quantity of lime in solution in a normal puer, in the form of calcium salts, appears to be about 1 grm. per litre. If more lime be presented to the liquor, the goods stop coming down. If now fresh dung be added, they will continue to come down, but the quantity of lime in solution does not increase; the excess of lime is precipitated, partly as phosphate and partly as oxalate, in the way stated above.

The type of reaction by which the lime is dissolved is similar to that occurring between ammonium chloride and lime, and is expressed by the formula--

2NH_{4}Cl + Ca(OH)_{2} = CaCl_{2} + 2NH_{3} + 2H_{2}O

and in the case of the calcium carbonate in the skins--

2NH_{4}Cl + CaCO_{3} = CaCl_{2} + (NH_{4})_{2}CO_{3},

so that every molecule of lime neutralized, liberates two molecules of ammonia.

Jean states that the production of ammonia progresses regularly during the puering, and that when the free ammonia reaches 0·2 grm. per litre the bate is unfit for further use. But if the excess of ammonia be neutralized by the addition of phosphoric acid, which also precipitates the lime brought in by the skins, the bate may be kept in constant use for some time. Jean found 0·06 grm. NH_{3} per litre in the puer as made up for use, and, after one lot of skins had been put through, 0·086 grm. per litre; after two lots of skins, 0·135 grm. per litre. In ordinary puer wheels, as used at Trent Bridge, we find--

(_a_) (_b_) Before goods 0·0816 0·0850 grm. NH_{3} per litre After " 0·0833 0·0799

so that there is little or no difference before and after puering in this case.

Part of the ammonia formed escapes into the air; a further portion unites with the acids which are being formed by the action of bacteria in the bate (see Chapter IV.), and it is also probable that part combines in some way with the calcium salts in the bate.

The actual reactions taking place are of course much more complicated than this simple case, because, instead of ammonia and ammonium chloride, we have a number of complex salts formed by combination of organic acids with substituted ammonia derivatives, such as methylamine, ethylamine, etc. The chief of these are ethylamine and methylamine butyrates, and lactates, and probably propionates, although I have not been able to separate the latter.[33]

[33] Fitz (Berichte, 1876–1884, see Herfeldt, J.S.C.I. May 31, 1895), has shown that lactates in dung are decomposed by bacteria, the chief product being propionic acid.

The reaction with the butyrate is represented by the equation--

2C_{2}H_{5}NH_{2}C_{3}H_{7}COOH + CaO

= 2C_{2}H_{5}NH_{2} + C_{3}H_{7}COO \ Ca + H_{2}O / C_{3}H_{7}COO

by which it will be seen that the amine is set free just in the same way as ammonia in the equation on p. 40.

In order to ascertain if the action of the amines was the same as that of salts of ammonia, I prepared the following compounds and tested their action on skins at 37° C., a control piece of the same skin being kept in water at the same temperature. The time in each case was one hour. All the solutions were neutral.

1. Ethylamine lactate: skin swells slightly, not “down” equal to puer.

2. Ethylamine propionate: skin moderately fallen, not equal to puer.

3. Ethylamine butyrate: about the same as Exp. 1.

4. Trimethylamine butyrate: very similar action to Exp. 3 and to ammonium butyrate.

The results correspond very closely with those obtained in previous experiments with various salts of ammonia (see Chapter VI.), and justify us in assuming that in all essential respects the action is similar. The free amines enter into fresh combinations with acids which are produced by bacterial action, and this process goes on until all the nutrient material is exhausted.

*The Role of Phosphates in the Bate.*--There is no doubt that the phosphates in the bate play an important part, but exactly in what manner they act is not yet known. One of the principal effects is the part they play as “buffers,” in preventing brusque changes of the hydrion concentration during the bating process. This has been pointed out by Soerensen in the case of enzyme reactions. The phosphates in dung are mixtures, which are capable of fixing both acids and bases; and so the small quantities of these bodies, which are produced by the splitting up of the organic matter, are taken up or released as the case may be.

The chemistry of the phosphates is one of the most complicated branches of inorganic chemistry, and, as a consequence, the determination of the constitution of the various phosphates in dung is an extremely difficult matter, and demands a lengthy research. For instance, besides the salts directly derived from the three phosphoric acids, HPO_{3}, H_{3}PO_{4} and H_{4}P_{2}O_{7}, phosphates exist which are probably derived from hypothetical di-, tri-, or meta-phosphoric acid, _n_HPO_{3}, and a few salts have been isolated, which are perhaps derived from the hypothetical acids P_{4}O_{7}(OH)_{6} and P_{10}O_{19}(OH)_{12} (Watt’s Dict., art. “Phosphates”). Including the double salts, there are more than 16 different calcium salts of phosphoric acids. The normal lime salt Ca_{3}(PO_{4})_{2} is very slightly soluble in water, but its solubility is increased by the presence of various organic substances such as exist in dung, and part of the soluble phosphates found in the bate are undoubtedly nothing more than this salt in solution. This fact has been utilized in the manufacture of the artificial bate Erodin (see Chapter VII.).

The phosphoric acid in the puer is partly precipitated by the lime in the skins, and hence diminishes during the bating process. In some cases practically the whole of the phosphoric acid disappears from the solution.

The following experiment will give an idea of the amount of lime precipitated as phosphate. A filtered puer liquor was analysed for lime and phosphoric acid, before and after the skins were passed through. The results were, in grm. per litre--

Before After Soluble lime as CaO 0·364 0·540 P_{2}O_{5} 0·424 0·328

Increase of lime, 0·176 grm.; diminution of P_{2}O_{5}, 0·096 grm. Calculated to calcium phosphate Ca_{3}(PO_{4})_{2}, this amount of phosphoric acid has combined with 0·114 grm. CaO. Assuming that the ratio of soluble to insoluble lime is the same as given, p. 36, then the lime is distributed as follows:--

Grm. Increase of soluble CaO 0·176 " " insol. CaO 0·210 Lime as Ca_{3}(PO_{4})_{2} 0·114 Hence " " oxalate (?) 0·096

That is, of the lime precipitated, 54 per cent. is phosphate, 46 per cent. oxalate.

In another puer containing before use 0·383 grm. P_{2}O_{5} per litre, only traces of phosphates were found in solution _after_ goods had been puered in the liquor, and in some analyses by Jean (39) the following figures were obtained:--

GRAMS OF PHOSPHORIC ACID PER LITRE.

Fresh dung after four days’ maceration 0·082 Bate after one pack of skins 0·036 Bate after two packs of skins 0·018

Although these figures are less than those found at Trent Bridge, they confirm the fact that the soluble phosphates diminish during the bating process.

The phosphates in solution thus diminish during the bating, and are found in the insoluble matter which separates out. A small portion of the lime remaining in the skins is also converted into phosphate by the action of the bate. In an experiment to determine this, a portion of the same skin was taken before and after puering. The pieces were dried, ashed, the ash dissolved in dilute nitric acid, and the phosphates precipitated by ammonium molybdate. In the skin before puering no phosphates were present, but in the skin after puering there was a small amount, though not sufficient to weigh.

The action of ammonium phosphate on the lime in the skin is very small. A skin was treated with a 0·1 per cent. solution of ammonium phosphate at 100° F. for one hour. The CaO in the dry skin was estimated, and found to be--

Per cent. Before the experiment 1·93 After " " 1·45

A considerable amount of calcium phosphate was found in the skin after the experiment.

Other chemical compounds existing in the puer, or formed by the action of bacteria (principally _B. coli commune_, see Chapter IV.), are indol, skatol and a number of aromatic oxyacids, principally para-oxyphenyl-propionic acid, a little para-oxyphenylacetic acid and skatol carbonic acid. In addition, tyrosin, leucin, tryptophan and mercaptans have been separated.[34]

[34] Rettger, Amer. Jour. of Physiol., viii. p. 284; Koch’s Jahresbericht, 1903, p. 112.

With these bodies no experiments on skin have been made, so far as I am aware, except with indol and skatol. Kathreiner found that these had a slight reducing action on skin, so that one may say they play some part in the puering.

The action of the bile salts, glycocholate and taurocholate of soda, also needs investigation. These have an indirect effect in the puer, as they favour the development of some species of bacteria (chiefly _coli_) and hinder the growth of others.

Some action has also been ascribed to sulphuretted hydrogen, but in the puer liquors which I have examined no H_{2}S was found, either before or after the skins were entered.

It will be seen that the ammonia compounds in the bate are not of themselves particularly fitted for the purpose of removing lime[35] from the skins, but owing to bacterial action (which we shall treat of in Chapter IV.), acids are produced which combine with the ammonia, and in this way the small quantity of these compounds originally present is continually being regenerated while the bating is in progress. Ammonia is set free by the lime in the skins. It is then neutralized by acids produced by bacteria, and thus acts as a carrier for the acids, and the bate remains in a nearly neutral condition. As the lime increases in the liquid the action of the bacteria diminishes, and finally the alkalinity becomes too great to allow the bacterial or chemical action to proceed further.

[35] Deliming with acids when carefully done takes out more lime than the puer, without in any way injuring the skin, but when tanned with sumac the bellies and axilla are harsh, and of a browner colour than in a puered skin. Analysis showed no more lime in the brown portions than in the centre of the skin, where the colour was quite bright and satisfactory.

It will be noted that the _concentration_ of the active salts in the bate is extremely small. If the amine compounds be assumed to consist of ethylamine butyrate or lactate, the concentration of the solution is approximately 1 grm. per litre; it is important that the concentration of salts should not greatly exceed this amount. I have found by experiment with ammonium chloride solutions, that the best reducing action is provided by a concentration of 0·7 to 1 grm. NH_{4}Cl per litre; if the concentration be raised to 2 or 3 grm. per litre, the skins become “leathery” and do not fall properly. The alkalinity must not be greater than 3–5 c.c. N/10 per 100 c.c. bate, for the bate to work at its best.

*Solution of Skin Substance during the Puering.*--The determination of the total skin substance dissolved by the puer is best done by Kjeldahl’s method before and after the goods.[36]

[36] Vide Procter, L.I.L.B., 1908, p. 64.

The difference in the total nitrogen found multiplied by 5·6 gives the amount of skin substance dissolved by the bate, assuming the amount of nitrogen in the dry ash-free skin to be 17·8 per cent. If very great accuracy be required, a small correction for nitrogen, brought into solution from the puer itself, is necessary.[37] This correction must be ascertained for the particular puer used by actual experiment.

[37] In a blank experiment to determine this, 0·0476 grm. N per litre was found to be brought into solution from the puer.

The following figures give the results in grams per litre obtained in the puering of sheep grains. 50 c.c. of the filtered puer liquor are slightly acidified with sulphuric acid, evaporated nearly to dryness and Kjeldahled in the usual way.

Mean Total nitrogen before skins {0·2632} 0·2604 {0·2576} " " after " {0·4928} 0·4844 {0·4760} Difference -- 0·2240

Equivalent to 1·254 grm. skin substance per litre. This was somewhat over a kilogram of skin substance for the paddle in question, and equal to 1 per cent. of the dry ash-free skin.

As to the differentiation of the dissolved skin substance into albumoses, peptones, monamino acids, diamino acids, ammonia, etc., a modification of Stiasny’s method[38] for the examination of soaks and old limes may be used.

[38] Stiasny, On Old Limes. Collegium 1910, p. 181.

The method is based on the fact, discovered by Schiff, that formaldehyde reacts with amino acids, forming methylen-amino acids, which are distinctly acid and allow a sharp titration with phenolphthalein as indicator, while the amino acids themselves react almost neutral. Soerensen has worked out a method on this basis for the determination of different amino acids, and for tracing the course of hydrolysis of albuminous matters.

Instead of using phenolphthalein as an indicator, the electrometric apparatus of Sand (see p. 76) is employed. 50 c.c. of the filtered puer liquor are put into a beaker, the hydrogen electrode is immersed in the liquor, and the potential difference (P.D.) observed; this gives the hydrogen ion concentration of the solution. 10 c.c. of neutral formaldehyde solution (40 per cent.) are added, and the P.D. again observed; it will be found to diminish rapidly, but soon becomes constant, indicating that the reaction is a quick one.

The increase of acidity, as shown by the lowering of the potential difference, is due to the acidity developed by the combination of the formaldehyde with the amino acids forming methylen-amino acids of appreciable hydrion concentration. The amount of such acids is estimated by titrating with N/10 caustic soda solution until the P.D. rises to the same voltage as that originally found. The following figures were found in an experiment:--

Original Puer The same Liquor After Goods π (volts) 0·61 0·69 π after 10 c.c. formalin 0·53 0·54 N (Kjeldahl), grm. per litre 0·3136 0·5936 Increase of N -- 0·2800 N/10 soda for 50 c.c. to original voltage 7·0 11·6 c.c. increase N/10 soda -- 4·6 ∴ 1 c.c. N/10 soda = mg N -- 3·05

A preferable method is to add decinormal acid or alkali to the original liquor until the P.D. of 0·69 is reached, at which point the liquor will be neutral to phenolphthalein, and, after adding formaldehyde, to titrate with N/10 soda until the P.D. of 0·69 is again reached.

The factor which connects the amount of decinormal soda required for the titration, after the addition of formaldehyde, with the total nitrogen as determined by Kjeldahl’s method, will afford information as to the extent of the hydrolysis undergone by the proteid matter, in the same manner as Stiasny (loc. cit.), has proposed to differentiate the dissolved proteid matter in lime liquors. As hydrolysis proceeds the percentage of nitrogen in the molecule increases, being at its maximum in the ultimate nitrogenous product ammonia; the factor, therefore, becomes less as hydrolysis becomes more advanced.

For ammonia 1 c.c. N/10 soda = 1·4 mg. N " hydrolized gelatin[39] = 2·9 " " Witte peptone completely hydrolized[39] = 3·6 " " lysin = 2·8 " " arginin = 5·6 " " histidin = 4·2 "

[39] Stiasny. Collegium 1910, p. 184.

We may conclude that the skin substance dissolved in the puer liquor is hydrolized almost as completely as gelatin is by boiling with sulphuric acid.

I have previously pointed out that dilute acids dissolve a certain amount of skin substance (see p. 157), and in this connexion, Dr. Georges Abt has given me the results of some experiments, on the solubility of skin in various organic acids, which he made in Vienna. Pieces of skin, weighing 40 grm. in the wet state, were allowed to remain for one month in N/10 solutions of the acids. The N was then determined, by Kjeldahl’s method, with the following results, expressed as per cent. of the wet skin dissolved:--

Per cent. Acetic acid dissolved 0·645 Lactic " " 2·27 Butyric " " 0·577 Formic " " 1·47

It will be seen that butyric acid dissolved the least amount of skin, lactic acid dissolving close upon four times as much.

*Scud.*--A certain amount of skin substance comes away in the “scud.” This is the liquid squeezed out of the skin by the pressure of the scudding knife after puering.

The liquid has the same composition as the puer liquor out of which the goods have been taken, and in addition contains large quantities of pigment granules, wool roots, and some skin substance, which together constitute the so-called “filth” of the skin. Analysis of a scud from English sheep grains showed only 0·164 per cent. N, equivalent to about 1 per cent. skin substance (9·15 grm. per litre). Fat, 7·9 grm. per litre.

Eberle and Krall have recently[40] analysed the fatty matter which adheres to the men’s knives in scudding lamb skins for gloving work. They obtained the following results:--

[40] Ueber die Zusammensetzung des beim Beizen von Lammfellen mit Hundekot abfällenden “festen Schmutzes”: ein Beitrag zur Beizenfrage.--Coll., 1911, p. 445.

Per cent. Water 29·7 Fat 42·0 Fatty acids combined with lime 6·6 Albuminous matter soluble in water 3·8 Hair and insoluble albuminous bodies 14·4 Ash (containing 57 per cent. CaO) 3·5

The fat had a--

Melting-point 40–44° Saponification number about 121 Iodine number 31·6 Hehner value 91·9 Acid value 9·3

Dr. Fahrion found, in a sample of the fat extracted with ether:--

Per cent. Iodine number. Unsaponifiable 47·6 27·3 Fatty acids (sol. in petrol ether) 39·3 30·2 Oxy acids (sol. in ether) 13·5 13·4

The figures obtained for the fat therefore agree closely with those for wool fat.

*Action of the Bird-Dung Bate.*--The depleting action of the pigeon- and hen-dung bate is very similar to that of the puer, or dog-dung bate; but the bating process with these materials, as we have seen (p. 18), is carried out at a lower temperature, and is consequently more prolonged. The principal difference between the two bates appears to be a chemical one, due to the fact that bird dung contains all the urinary products which are present only to a small extent in the dung of mammals. In birds uric acid is the chief stage in nitrogenous katabolism, the mechanism of its formation being a process of synthesis in the liver (Halliburton). Urea is also present in considerable amount, and does not appear to be so easily decomposed as the urea in animal urine.[41] As we shall show in the next chapter, urea, and probably also urates, greatly facilitate the permeability of gelatine, and to this fact may be ascribed the more gradual action of bird-dung bates. If we attempt to bate hides with dog-dung, the grain of the hide is found to be attacked and destroyed before the bate has penetrated to the interior of the hide. On the other hand, a bird-dung bate may be used at a temperature of 38° to 40° C. for the puering of skins destined for light leather, but its action is not so favourable as that of the puers.

[41] Urea is present to the extent of 5 per cent. in samples of Peruvian guano.

COMPOSITION OF BIRD EXCRETA.

------------------------+---------------+-------+-------+------ | Pigeon | Hen | Duck | Goose ------------------------+---------------+-------+-------+------ Moisture | 58·32 | 56·08 | 60·88 | 46·65 | 77·08 Organic matter† | 28·25 | 19·56 | 19·22 | 36·12 | 13·44 Phosphates | 2·69 | 2·54 | 4·47 | 3·15 | 0·89 Carbonate and sulphate} | 1·75 | 3·08 | 7·85 | 3·01 |} of calcium } | | | | |} 2·94 Alkaline salts | 1·99 | 0·82 | 1·09 | 0·32 |} Silica and sand | 7·00 | 17·92 | 6·69 | 10·75 | 5·65 ------------------------+-------+-------+-------+-------+------ |100·00 |100·00 |100·00 |100·00 |100·00 ------------------------+-------+-------+-------+-------+------ † Containing nitrogen } | | | | | equal to ammonia } | 1·75 | 1·21 | 0·74 | 0·85 | 0·67 ------------------------+-------+-------+-------+-------+------

Macadam[42] states that pigeon dung is the most concentrated. Hen manure contains the largest proportion of phosphates, and is followed by duck droppings. That of the goose is the least valuable. The preceding table is taken from his paper.

[42] Manures, Natural and Artificial, W. Ivison Macadam. Jour. Soc. Chem. Ind. 1888, p. 79.

Procter[43] quotes the following, as a mean of 40 analyses of pigeon dung by Schulze:--

[43] Principles of Leather Manufacture, p. 179.

Per cent. Water 21·00 Nitrogen 2·53 Phosphoric acid 1·79 Potash 1·46

He remarks that the action of bird dung is more penetrating, but less softening and loosening than that of dog dung, and this effect may be explained by what has been said above.

Unfortunately, far less work has been done on the bird-dung bate than on the puer, and there is a wide field open for research in this direction.