CHAPTER II.

THE CHEMISTRY OF BATING.

“And now, as we cannot understand the _frame_ of a _Watch_, without taking it into pieces; so neither can Nature be well _known_, without a _resolution_ of it into its _beginnings_, which certainly may be best of all done by _Chymical Methods_”--JOS. GLANVILL, 1668.

Professor H. R. PROCTER, in his “Principles of Leather Manufacture,” 1903, p. 153, has given a very complete account of chemical deliming, and also of the bating and puering processes. Meunier and Vaney, “La Tannerie,” 1903, give a general review of our knowledge of these processes up to that date. These accounts are extremely useful, but treat the subject in a general manner.

I propose to treat of the particular case of puering by means of an infusion of dogs’ dung in water, as I believe this will give an insight into all the other bating processes.

So far as the purely chemical action of the bate goes, it consists in the solution of the lime contained in the skins,[18] thus setting free a certain portion of skin substance which was combined with the lime, and the subsequent solution of more or less of this skin substance.

[18] Limed sheep grains, or Pelts, which have been limed for splitting, contain from 3 per cent. to 6 per cent. CaO on the dry skin (see footnote, p. 7), and about 80 per cent. of water. In a typical case of a fully limed grain, the lime was found to be distributed as follows:--

Free uncombined lime (CaO) 1·7 per cent. on the dry skin CaO combined with skin 2·5 " CaO as carbonate, or other salts 1·4 " ----- 5·6

Thus, 75 per cent. of the total lime in the skin was in the caustic state. If the skin be cut during the progress of the bating operation, and a solution of phenolphthalein applied to the cut surface, the course of bating may be followed, and it will be found that the caustic lime disappears from the skin in a very short period of time, accompanied by the sudden collapse of the fibres. We shall treat of this more fully in Chapter III. After puering, the lime (CaO) contained in the skin, amounts to 0·5 to 0·9 per cent., calculated on the dry skin. This lime is in a perfectly neutral state, and is more or less in combination with the fibre of the skin. The exact state in which the lime exists in the skin _after_ puering is not known, and would form a very interesting subject for research.

If a fresh puer be made and boiled for half an hour, then allowed to cool to 95° F., it will be found to remove the lime from the skin in a very similar way to an ordinary bate, but it has not so rapid a reducing action as an unboiled bate. In this case both bacteria and enzymes are destroyed, so that the action may be put down to the chemical constituents of the bate.

*Chemical Composition of the Puer.*--The mineral constituents of the fæces, both in dogs and human beings, are well known, owing to the study of the processes of digestion and nutrition in physiological laboratories; but the organic constituents are yet little known, and the sum of the weight of those at present estimated is far from the total of these matters present.

It is a mistake to suppose that the fæces represent the residue only of the food taken. Strassburger[19] estimates that bacteria alone account for nearly one-third of the dry matter. In addition, the intestinal mucous membrane is an important excretory channel for lime, magnesia, iron and phosphoric acid, as has been shown by analyses of fæces from men and dogs, during prolonged fasting.[20] Even when no nitrogenous food is eaten, the dry fæces always contain from 4 to 8 per cent. of nitrogen; in a dog fed on meat it amounts to 6·5 per cent.

[19] Schmidt, and Strassburger, Die Fæces des Menchen (Hirschwald, Berlin), 1901.

[20] Lambling, Précis de Biochimie, 1911, p. 221.

In making a chemical analysis of the fæces, the mineral matter is estimated in the ash in the usual way for ash of organic substances, with special precaution, on account of the phosphates present. The material is first charred, the salts removed by acetic acid; the acetic acid solution is then decanted, the residue washed with distilled water, and the combustion then completed. The acetic solution and washings are added to the final ash, the whole evaporated to dryness, and gently ignited to decompose the acetates. (Cf. Bull. 46, U.S. Dept. of Agriculture, Washington, 1899.)

In estimating the fats, the dry puer is ground up with sand, then extracted with ether in a Soxhlet. The fats are present in four states:--1, neutral fats; 2, free fatty acids; 3, alkali soaps, soluble in ether; 4, small quantities of lime and magnesia soaps remaining in the residue in the Soxhlet. For details as to the separation and estimation of these, Lewkowitsch’s “Oils, Fats and Waxes” (Macmillan) should be consulted.

The separation and estimation of the organic constituents in the puer, is one of the most difficult problems of physiological chemistry, and to include the methods used would demand a special treatise beyond the scope of the present volume. Hoppe-Seyler’s “Physiological Chemistry,” and Allen’s “Commercial Organic Analysis,” vol. iv., may be consulted; and there is a mass of useful information in Dr. René Gaultiers’ “Précis de Coprologie Clinique” (Paris, Baillière et Fils, 1907).

It is proposed to give here the results of a number of analyses of dog dung made by the author and others, and to discuss the action of the various bodies upon the skins. Many more experiments and much research requires to be done, before the _complete_ action of the bate is clear, but only by the method of experiment with the various bodies present can the problem eventually be solved.

Analysis of raw puer from hunting kennels (Quorn) showed the approximate composition of 1000 grm. of raw puer, containing 150 grm.[21] of dry matter to be as follows:--

[21] Another sample contained 136 grm. dry matter.

Grammes

Sodium chloride and sodium sulphate 2·1 Sodium ammonium phosphate 14·0 Earthy phosphates, principally Ca_{3}(PO_{4})_{2} 33·6 Ferric phosphate 0·87 Calcium sulphate 1·94 Silicic acid 3·40 Calcium in solution as CaO 1·42 Total P_{2}O_{5} in solution 4·00 Non-volatile acids (as lactic) 3·00 Volatile acids (as acetic) 2·20 Amines (as ammonia) 6·20 Enzymes 3·66 Nitrogenous bodies not included in the above, consisting of complex amido-acids, leucin, tyrosin, xanthin, and other purine bases, indol and skatol 17·00 Organic matter, cellulose, etc. mostly insoluble, including fats and soaps 70·00

On the dry matter the amounts extracted by various solvents were:--

Per cent. Water 24·03 Alcohol 22·27 CS_{2} 15·57 C_{2}HCl_{3} (Tri-chlorethylene) 14·45 Petrol. ether 13·23

No carbohydrates (sugars) capable of reducing Fehling’s solution were found in the puers examined.

The ash[22] of the dry fæces from dogs fed on a flesh diet varies from 20 to 34·27 per cent., with the following percentage composition:--

[22] Gaultier. See also analysis given in Chapter VI.

Silica 7·04 CO_{2} 4·62 SO_{3} 7·37 Fe_{2}O_{3} 4·22 CaO 25·29 P_{2}O_{5} 26·41 MgO 15·52 Cl 1·50 Alkali 5·53

In a dog weighing 30 kilos., fed on a mixed diet of 500 grm. meat and 200 grm. starch, 78·6 grm. dry fæces were obtained containing 23·76 per cent. ash of the following percentage composition:--

CaO 22·3 P_{2}O_{5} 25·4 Fe_{2}O_{3} 10·6 MgO 9·8 SO_{3} 5·0 Cl 0·2 Alkali 1·1 Insol. in HCl 21·8

It is obvious that the composition of the fæces will vary with the food given, and in this connexion it may be stated that the puer used was obtained from dogs fed on a mixture of boiled horseflesh and oatmeal porridge, about equal quantities. Sometimes charcoal and cabbage are mixed with the food, in this case the puer is very dark coloured. The dark colour so produced is to be carefully distinguished from the dark colour caused by decomposition of the puer.

We cannot leave out of account the urinary products, which, in the case of dung from hunting kennels, are always present, though in varying quantities.

The total nitrogen in 1520 c.c. of urine was found[23] to be 15·9 grm.

[23] Abderhalden, Physiological Chemistry.

Each 100 grm. total nitrogen was distributed as follows:--

Urea 85·9 NH_{3} 4·1 Creatinine 3·3 Uric acid 0·5 N in other compounds 5·7

The inorganic compounds were:--

Per cent. Sulphur (as sulphate) 3·31 P_{2}O_{5} 3·98 Cl 6·30

The urine of carnivora is acid, that of herbivora is neutral or alkaline.

Urea, which is the chief nitrogenous constituent, is very quickly fermented by several species of bacteria, and entirely decomposed into ammonium carbonate, so that it is this latter body which acts in the bate.

Uric acid exists in the urine of the carnivora, but the quantity is small compared with the urea. Herbivora excrete, mostly, hippuric acid

NHC, H_{5}O / (benzoyl-glycocoll, CH_{2} ), \ COOH

and this difference may account to some extent for the difference in bating property, that of the excreta of herbivora being very small as compared with the carnivora, such as the dog. In this respect there are differences which are difficult to account for; for instance, the excreta of the lion, fed entirely on a meat diet, was not found to have such a powerful bating effect as ordinary dog puer. Even after keeping for several weeks, to allow the bacterial flora to develop, the result was the same.

The analysis of the lion’s dung gave the following results:--

Per cent. Water 59·2 Ash 21·1 Lime (CaO) 10·3 Phosphates (P_{2}O_{5}) 10·67 Organic matter 19·7

Dog dung contains some non-volatile bases of the purine group, such as xanthin (C_{5}H_{4}N_{4}O_{2}) and guanin (C_{5}H_{5}N_{5}O), but the action of these bodies in the bate, has not been studied. It is also extremely probable that these and similar bodies are formed _during_ the bating by a process of auto-digestion of various proteids.[24] The quantity of _fat_ in dry puer, amounts to 10–11 per cent. Cholesterol was present.[25] Part of the fat is in the form of lime and magnesia soaps, but part in the form of an emulsion, which probably plays an important part in the puering process. The function of the fatty constituents of puer is one of the problems on which further research is necessary.

[24] Cf. Mann. Chemistry of the Proteids, 1906, p. 432.

[25] The cholesterol was obtained by extracting the dry puer with ether, saponifying with alcoholic potash. Shaking out the unsaponifiables with ether, evaporating to dryness. The residue was taken up with absolute alcohol, decolorized with animal charcoal. On cooling, characteristic crystals of cholesterol separated. Hoppe-Seyler finds from 1 per cent. to 4 per cent. cholesterol in fæces. See also Gaultier, Coprologie Clinique, p. 160.

The colouring matters of the puer are nearly all derived from bile products. Most of them are found in the petroleum extract, which contains far more of the colouring matters than the chloroform extract.

The following bile colouring matters have been prepared by Merck, of Darmstadt: bilihumin, biliprasin, bilirubin, bilifuscin, biliverdin.[26] So far as has been ascertained by the author, their effect in the bating is nil, but, in so far as they impart colour to the pelt, they are detrimental.

[26] For qualitative tests for the colouring matters, see Gaultier, Précis de Coprologie Clinique, p. 159.

Experiments on the action of bile are given in the paper reprinted in