I have shown[128] that simple cultures of bacteria in certain nutrient media, in the absence of amines and other bodies, have an imperfect bating action on skin. This was the case, whether the cultures were pure or mixed, but the mixed cultures have a better action than the pure cultures of any of the organisms I have hitherto used. Bacteriological processes in nature are usually carried on by a mixture of species of bacteria; still there are fermentations like the spontaneous souring of milk, the formation of nitrites from nitrates, and the ammoniacal fermentation of urea, which may be considered as natural pure cultures. These are examples of what I may call the selective influence of the nutrient medium. The nutrient medium used in my former experiments was a solution of gelatin and mineral salts, such as is used in general bacteriological work, modified in various ways. On careful consideration, this did not appear to be a medium at all corresponding to the natural bate. The albuminoids in the process of digestion are peptonised, and dung contains only those bodies which the animal is incapable of assimilating. It is from these bodies that the necessary enzymes are produced by bacteria. Of the numerous trials with pure cultures of different organisms from dung, I found that most of those having the property of secreting bating enzymes, were non-liquefying bacteria. Popp and Becker have also pointed out that peptonising bacteria do not exert a favourable reducing action on skin. It is impracticable to grow these non-peptonising organisms on a large scale in a solid medium, and it occurred to me that gelatin, previously peptonised by chemical means, would be a favourable medium. Experiments with hydrochloric and sulphuric acids gave hopeful results. The acid was neutralised with ammonia after the chemical action was complete, and the bacteria cultivated in this solution. [128] J.S.C.I., 1898, p. 1011, see previous paper, p. 152. A better result was obtained by digesting 10 grm. gelatin, 5 grm. lactic acid (anhydrous by calculation), 100 c.c. of water, in a closed vessel on the water bath for three hours. A slight black precipitate of melanoidic acid (humic acid) is formed. The liquid is of a clear brown colour, and contains a large number of nitrogenous bodies resulting from the breaking down of the gelatin molecule. It has been impossible for me to ascertain its exact composition, but I have partially examined it with the following results:-- 1. A small quantity of the liquid poured into absolute alcohol, gives a white precipitate which re-dissolves on the addition of about 30 per cent. water. This shows absence of gelatin.