Victor Henri has shown that the power of metals in the colloidal state to bring about these catalytic actions varies with the metal employed, and is in inverse ratio to the size of the particle. There is a very interesting and wide field of research open here in order to determine the conditions under which the various metals act. To this end the ashes from the purest enzyme preparations might be studied, and methods devised for producing these metals in the colloidal state, for it seems evident that it is the _state_ of the body acting which gives it the properties observed, and not its chemical properties in the usual sense. The enzymes contained in dog dung which are effective in the puering process belong to several groups, principally the proteolytic and lipolytic groups, but indirectly enzymes of the first group (see p. 132), (fermenting carbohydrates) and of the fifth group (fermenting urea) also play some part by decomposing various compounds (e.g. cellulose and urea) contained in the puer. The action of certain enzymes from the animal body upon skin has been tried by the author.[103] Those selected were pepsin and pancreatin,[104] as being most likely to be present in dung. Pepsin only acts in presence of HCl. Two portions of the same skin were taken, one of them was treated with a 1 per cent. solution of pepsin, acidified with 0·2 per cent. of hydrochloric acid; the other in a bate liquor of dogs’ dung (puer), both at a temperature of 40°C. At the end of one hour the skin in the pepsin solution was considerably “fallen,” but that in the manure solution was bated nearly away, i.e. the greater part of it was dissolved. A 1 per cent. solution of pancreatin (Mercks) was found to act far more rapidly than pepsin; 1·5 per cent. of chloroform was added to the solution, to prevent the development of bacteria. The skin was reduced, but had not the peculiar touch of a puered skin. As will be shown later, this was found to be due to the absence of any chemical action upon the lime salts in the skin, and consequently it felt “limey.” This action took place in the puered skin, but not in the skin treated with pancreatin alone. [103] J.S.C.I., vol. xiii., 1894, p. 218. [104] The word pancreatin is used throughout in the sense of pancreas extract. As is well known, this contains several enzymes, trypsin, steapsin, maltase, and also a rennin. W. J. Salomon (9) has also attributed the activity of the bate to pepsin and pancreatin, but he does not give any proof of the presence of these ferments in the bate. Since it is practically impossible up to the present, to separate these enzymes from the dung in a state of purity, the method described on p. 134 was employed.