Per cent. Water 21·00 Nitrogen 2·53 Phosphoric acid 1·79 Potash 1·46 He remarks that the action of bird dung is more penetrating, but less softening and loosening than that of dog dung, and this effect may be explained by what has been said above. Unfortunately, far less work has been done on the bird-dung bate than on the puer, and there is a wide field open for research in this direction. CHAPTER III. THE PHYSICS OF BATING. “The most important aspect of any phenomenon, from a mathematical point of view, is that of a measurable quantity.” --MAXWELL. It is impossible in the space of a short chapter to give an adequate explanation of the physical changes taking place during the puering process. An outline only can be given, and perhaps a few signposts to indicate for those interested, in what direction further work can be usefully done. It is obvious that if more be attempted, a treatise on physics and physical chemistry would be required; such a work is beyond the scope of the present volume. It is to be hoped that the whole of the questions dealt with here, including the physical chemistry of skin and of the whole tanning process, will shortly be fully treated by the master hand of our greatest tanning chemist, Professor H. R. Procter. Since tanning, in the earlier stages, is for the greater part a physical absorption, or colloidal co-precipitating action,[44] the physical state of the skin fibre, or condition of the skin before it enters the tanning liquor, is of the greatest importance. In fact, the whole of the operations to which the skin is subjected previous to tanning are directed towards changing its physical state, and the chemical changes undergone are small and principally hydrolytic. [44] Stiasny, Kritische und Experimentelle Beiträge zur Aufklärung der Gerbvorgänge. Collegium 1908, p. 117. Wood, Compounds of Gelatin and Tannin. Collegium 1908, p. 494. The two most important physical influences over which we have control are pressure and temperature. The pressure during puering is practically constant, viz. the atmospheric pressure, but, as we shall see later, a diminution of pressure is favourable to falling or depletion. It would, therefore, be interesting to conduct experiments on puering in vacuo, or under reduced pressure. Before considering the effect of temperature and the changes in the volume of the skin during puering, it will be well to consider the properties of the skin and the puer liquor. In the skin prepared for tanning, practically all the keratin and epidermis and soluble matters are got rid of, and we have a mass of fibres which are composed of collagen, a proteoïd, which by prolonged boiling with water is converted into gelatin.