The bacteria of fresh dog-dung were not found to possess a satisfactory puering effect, but those from dung which had been fermented a month (as in practice) gave a result nearly equal to actual puer. A still better result was obtained by a mixed culture from the roots of wool loosened by sweating. The bacteria were principally of two species, of which neither separately was capable of satisfactory puering; but which together acted more rapidly than an actual puer. These bacteria do not liquefy gelatine. During the course of his experiments, Wood found that filtered puer solutions were less active than turbid ones and that their activity was increased even by the addition of inert substances, such as kaolin. Wood attributes the differences in action between dog-dung and bird-dung not only to different bacteria, but to the fact that in the latter case the urinary products, and especially uric acid are contained in the dung. From the results of these and similar researches, Wood in England, and Popp and Becker in Germany succeeded in producing a practical artificial puer, which they now manufacture in conjunction under the name of “Erodin.” “Erodin” consists of a solid nutrient medium and a liquid “pure culture” of the bacteria necessary to effect the required bating or puering. The following are the directions for working with erodin bate, as supplied by the manufacturers:-- “For 100 lb. of wet skin washed ready for bating, about 1 lb. of erodin is required. Or in the metric system, 1 kilo. wet skin requires about 10 grm. erodin. The strength or concentration of the bate must not fall below 3 grm. per litre of bate liquor, i.e. ¹⁄₂ oz. per gallon. For preparing the bate a sufficiently large cask or tub carefully _cleaned_ and steamed out is placed near the bating paddle. The cask should be fitted with a steam pipe easily screwed on and off, and also furnished with a _clean_ cover. The requisite quantity of erodin is weighed out and put into the tub with fifty times its weight of water, and the whole brought up to a temperature reaching but not exceeding 40° C. (104° F.) by direct admission of steam, thoroughly stirred, and the pure culture of _Bacillus erodiens_ added to the mixture. The temperature must not be allowed to fall below 25°C. (87° F.), and a little steam should be admitted first thing in the morning, again at noon, and in the evening, to bring the temperature up to 40° C. (104° F.).