THE CHEMISTRY OF BATING.

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.

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. Strassburger19 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.

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 1000grm. of raw puer, containing 150 grm.21 of dry matter to be as follows:—

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

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

The ash22 of the dry fÆces from dogs fed on a flesh diet varies from 20 to 34·27 per cent., with the following percentage composition:—

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

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 1520c.c. of urine was found23 to be 15·9grm.

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

The inorganic compounds were:—

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.

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

Dog dung contains some non-volatile bases of the purine group, such as xanthin (C₅H₄N₄O₂) and guanin (C₅H₅N₅O), 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.

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.

Experiments on the action of bile are given in the paper reprinted in Chapter VI.

Recently Eberle27 has proposed to add a proportion of gall juice (bile) to a bate containing pancreatic enzymes, with a view of energizing the latter. This part of the subject will be dealt with in Chapter V.

The Reactions of the Puer.—The organic acids are present principally in the form of the sodium or calcium salts. The intestinal juices contain sodium carbonate, but this is neutralized by lactic acid found in the intestines, so that the excreta usually have an acid reaction. The reaction of a fresh puer liquor is faintly acid to litmus, but this acid is immediately neutralized by the introduction of the skins, so that the rest of the bating process takes place in a neutral or alkaline medium, and the quantity of lime removed by the free acids is probably small.

The acidity of a filtered puer liquor made up freshly and titrated with N/5 soda, and phenolphthalein as indicator, was found to equal 10c.c. N/1 acid per 1000c.c. of bate. In the volume of liquor used, namely, 900 litres, this would neutralize only 250grm. of lime (CaO).

The determination of the exact acidity or alkalinity of puer liquors by ordinary methods with indicators is not easy, as the end reactions are not sharp. The liquor above described was alkaline to methylorange and litmus, although acid to phenolphthalein, and the results also vary somewhat, according to the quantity of bate used for titration, and according to the strength of the acid or alkali used. The method used, both at the Trent Bridge laboratory and Messrs. Doerr and Reinhart’s laboratory at Worms, is as follows:—

The puer liquor is filtered through a Schleicher and Schulls hard filter, No. 602, 18 1/2cm. diam.; 50c.c. are taken, 4 drops of phenolphthalein solution (10gr. dissolved in 300c.c. alcohol) added, and titrated with N/5 acid or alkali as the case may be.

If the acidity be determined by adding excess of alkali and titrating back, a larger amount of acid is found than by direct titration. In the above-mentioned case, an indirect titration showed an acidity equal to 18c.c. N/10 acid per 100c.c. of liquor.

On adding the alkali to a fresh filtered puer liquor, a flocculent precipitate is produced, which appears to be due to the decomposition of proteid compounds of weak organic acids,28 the alkali uniting with them and setting the proteids free. As these compounds are undoubtedly decomposed by the lime in the skins, it seems probable that the acidity available for neutralizing lime may be greater than that shown by direct titration.

A method which was suggested by me for the estimation of the acidity of tan liquors may be applied also to the examination of the bate liquors, namely, the method by which the electric potential between a hydrogen electrode dipping into the bate liquor, and a standard calomel electrode, is used to determine the neutral point (for details see Chapter III.), using the potentiometer devised by Dr. H.J.S. Sand.29

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 100c.c. filtered liquor.

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.

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. 7–8c.c. N/10 acid per 100c.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 1000c.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:—

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 = 38c.c. N/1 acid, and for 3 and 4, 51c.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 28grm. of lime, it follows that the lime removed by the bate was in one case 862grm., in the other case 1157grm.

The 500 kilos, of wet washed grains introduced into the paddle contain approximately 0·5 per cent. of lime CaO, equivalent to 2500grm. 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 10×810c.c. of normal alkali, and this is equivalent to 225grm. of CaO, or 9 per cent. of the total lime.30

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.

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 0·295grm. per litre of soluble lime, and 0·355grm. per litre of insoluble lime, a total increase of 0·650grm.; 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 Zopf31 and Banning32; but it is difficult to estimate it in the bate, and a reliable analytical method of doing so requires devising.

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:—

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.

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:—

The limit of the quantity of lime in solution in a normal puer, in the form of calcium salts, appears to be about 1grm. 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—

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

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 0·2grm. 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 0·06grm. NH₃ per litre in the puer as made up for use, and, after one lot of skins had been put through, 0·086grm. per litre; after two lots of skins, 0·135grm. per litre. In ordinary puer wheels, as used at Trent Bridge, we find—

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

The reaction with the butyrate is represented by the equation—

2C₂H₅NH₂C₃H₇COOH + CaO

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.

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, HPO₃, H₃PO₄ and H₄P₂O₇, phosphates exist which are probably derived from hypothetical di-, tri-, or meta-phosphoric acid, nHPO₃, and a few salts have been isolated, which are perhaps derived from the hypothetical acids P₄O₇(OH)₆ and P₁₀O₁₉(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 Ca₃(PO₄)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—

Increase of lime, 0·176grm.; diminution of P₂O₅, 0·096grm. Calculated to calcium phosphate Ca₃(PO₄)₂, this amount of phosphoric acid has combined with 0·114grm. CaO. Assuming that the ratio of soluble to insoluble lime is the same as given, p.36, then the lime is distributed as follows:—

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

In another puer containing before use 0·383grm. P₂O₅ 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.

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—

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

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 H₂S 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 lime35 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.

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 1grm. 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 1grm. NH₄Cl per litre; if the concentration be raised to 2 or 3grm. per litre, the skins become “leathery” and do not fall properly. The alkalinity must not be greater than 3–5c.c. N/10 per 100c.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

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.

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

Equivalent to 1·254grm. 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 method38 for the examination of soaks and old limes may be used.

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. 50c.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. 10c.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:—

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.

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 40grm. 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:—

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 (9·15grm. per litre). Fat, 7·9grm. per litre.

Eberle and Krall have recently40 analysed the fatty matter which adheres to the men’s knives in scudding lamb skins for gloving work. They obtained the following results:—

The fat had a—

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

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.

Composition of Bird Excreta.

Macadam42 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.

Procter43 quotes the following, as a mean of 40 analyses of pigeon dung by Schulze:—

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.