*General Considerations on the Growth of Bacteria in Various Media.*--Since the publication of Further notes on the action of the dung bate (Chapter VI.), I have found that the bating organisms grow better in the special medium, when it is neutralized with ammonia, than when it is neutralized with sodium carbonate, i.e. the presence of organic ammonium salts is more favourable to the growth of the bacteria than the corresponding sodium salts. I also found that bacteria obtained from other sources than dung, viz. from the roots of wool just beginning to “slip” in a sweating stove, were equally effective in causing the skin to fall. Now these bacteria produce ammonia, and it seems clear that they are essential to the chemical part of the process. They also produce proteoclastic enzymes, which act upon the skin fibre (see chapter on Enzyme Action). The products of the bacteria depend very much upon the composition of the nutrient medium. Many organisms grown in media containing sugar or other carbohydrates produce acids, but, grown in proteids free from sugars, they produce alkaline compounds. Villon (“The Leather Industry,” 1901, p. 408) describes a bacterium which he considers to be the special micro-organism concerned in the depilation of skins, which resembles Bacillus _d_ (Wood) (Fig. 21), but he does not describe the appearance of the cultures; he states, however (p. 410), that this is the only bacterium which can develop in the limes,[83] and that it is the cause of the unhairing in this case also. Since the production of ammonia in limes is known to be due to bacterial action, it is very probable that this bacterium, which is ubiquitous, is also of use in the bate, and a research in this direction would be interesting.[84] [83] Villon found that Bacterium pilline developed 0·142 per cent. ammonia in ten days in lime liquors used for unhairing skin. [84] Cultures made from old limes have, however, not been found effective in bating. Some of the fermentations taking place in the dung come under the heading of putrefactive processes (see p. 116). Tyrosin is formed in considerable quantities during putrefactive fermentation, but is soon further decomposed, according to Nencki, with formation of indol, CO_{2} and hydrogen. Leucin gives valerianic acid, ammonia, CO_{2} and hydrogen; nitrogenous bodies of the aromatic series are also produced. [Illustration: Fig. 21.--Bacillus _d_.] [Illustration: Fig. 22.--Bacillus _e_.] Bacillus ureaæ, B. prodigiosus, and B. fluorescens putridus, evolve trimethylamine (Herfeldt), and, as the writer has shown, this amine has an important action in the puering process. In combination with organic acids, it removes lime from the skin, and in addition it favours the growth of bacteria, such as bacillus _d_ and _e_ (Figs. 21 and 22) and B. coli.