It is a mistake to suppose that the fæces represent the residue only of the food taken. Strassburger[19] estimates that bacteria alone account for nearly one-third of the dry matter. In addition, the intestinal mucous membrane is an important excretory channel for lime, magnesia, iron and phosphoric acid, as has been shown by analyses of fæces from men and dogs, during prolonged fasting.[20] Even when no nitrogenous food is eaten, the dry fæces always contain from 4 to 8 per cent. of nitrogen; in a dog fed on meat it amounts to 6·5 per cent. [19] Schmidt, and Strassburger, Die Fæces des Menchen (Hirschwald, Berlin), 1901. [20] Lambling, Précis de Biochimie, 1911, p. 221. In making a chemical analysis of the fæces, the mineral matter is estimated in the ash in the usual way for ash of organic substances, with special precaution, on account of the phosphates present. The material is first charred, the salts removed by acetic acid; the acetic acid solution is then decanted, the residue washed with distilled water, and the combustion then completed. The acetic solution and washings are added to the final ash, the whole evaporated to dryness, and gently ignited to decompose the acetates. (Cf. Bull. 46, U.S. Dept. of Agriculture, Washington, 1899.) In estimating the fats, the dry puer is ground up with sand, then extracted with ether in a Soxhlet. The fats are present in four states:--1, neutral fats; 2, free fatty acids; 3, alkali soaps, soluble in ether; 4, small quantities of lime and magnesia soaps remaining in the residue in the Soxhlet. For details as to the separation and estimation of these, Lewkowitsch’s “Oils, Fats and Waxes” (Macmillan) should be consulted. The separation and estimation of the organic constituents in the puer, is one of the most difficult problems of physiological chemistry, and to include the methods used would demand a special treatise beyond the scope of the present volume. Hoppe-Seyler’s “Physiological Chemistry,” and Allen’s “Commercial Organic Analysis,” vol. iv., may be consulted; and there is a mass of useful information in Dr. René Gaultiers’ “Précis de Coprologie Clinique” (Paris, Baillière et Fils, 1907). It is proposed to give here the results of a number of analyses of dog dung made by the author and others, and to discuss the action of the various bodies upon the skins. Many more experiments and much research requires to be done, before the _complete_ action of the bate is clear, but only by the method of experiment with the various bodies present can the problem eventually be solved. Analysis of raw puer from hunting kennels (Quorn) showed the approximate composition of 1000 grm. of raw puer, containing 150 grm.[21] of dry matter to be as follows:-- [21] Another sample contained 136 grm. dry matter. Grammes