With a strength of 1 grm. per 100 c.c. the action was hastened, the same result being obtained in 1-1/4 hours, but the coriin of the skin was not dissolved. A comparison piece of the same skin in dung was bated too much. With a view of comparing the action of analogous bodies, skin was treated in a solution of anilin hydrochloride, 1 grm. per 100 c.c. for 1-1/4 hours at 35° C. The solution was acid, lime was removed, and the skin felt very similar to that in the previous experiment. Coriin was not dissolved, and the skin did not “come down.” Of the mineral constituents mentioned above, only the chlorides[116] have a reducing action on skin, the other compounds being inert, e.g. silicates, or merely supply food for the bacteria. [116] Procter, Text Book of Tanning, p. 85. For a general outline of the decompositions taking place in dung, see Dr. Herfeld’s paper (J.S.C.I., May 1895). We need only consider the action of the chief organic compounds, as the amount of chlorine is too small (in one case 0·053 per cent. on the dry dung) to be of importance. The principal compounds are:-- _Organic Acids._--Formic, acetic, butyric, valerianic, lactic, malic, tartaric, citric, and glyceric. _Amido Compounds._--Leucin, tyrosin, glutamic acid, glutamine, asparagin, glycocol. _Bases_, consisting of amines, skatol, and indol, and varying amounts of ammonia, according to the age of the dung. The organic acids exist partly as salts of the alkaline earths, partly combined with amines. A distillation of “puer,” after acidifying with H_{2}SO_{4}, gave 2·2 grm. per litre of volatile acids reckoned as acetic. No HCl came over. On decomposing the sodium salts of the acids with H_{2}SO_{4}, the smell of butyric acid predominated. I have also shown the presence of lactic acid in the dung, but have been unable to determine the amount. _Action of Amido Compounds._--A mixture of glycocol and leucin[117] was prepared by boiling 8 grm. of gelatin in 400 c.c. of water, acidified with 1 c.c. of HCl for two hours with inverted condenser. The acid was then neutralised with ammonia. The solution had a very considerable reducing action on skin at 35° C. in 90 minutes, but not sufficient for practical purposes. It appears to dissolve a little coriin in a similar way to dilute acids. It was found on further investigation that the action was largely due to the NH_{4}Cl present. [117] According to the text books; but for later researches see Paal and Schilling, Chem. Zeit. 1895, 1487; also J.S.C.I., 1898, 589. _Action of Dilute Acids._--Although the bate in practice is almost always alkaline, yet a fresh bate is _acid_, and it may be as well, before proceeding, to consider briefly the action of weak and dilute acids on skin.