| 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]
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 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.
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. NH4Cl 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]