Müller[60] points out that jellies containing certain non-electrolytes (glucose, glycerine, alcohol) render the diffusion of soluble matter much slower than pure jellies of the same concentration, but the presence of urea favours the permeability of gelatin and agar jellies. The differences in the velocity of diffusion of various salt solutions through jellies of the same strength, are thus not due altogether to the greater diffusibility of the particular salt solution, but are to be attributed to the influence of the salt on the permeability of the colloidal medium. Limed skin treated with a 1 per cent. solution of urea at 43° C. is depleted or “falls,” but both the solution and the skin remain alkaline. [60] Allgemeine Chem. d. Kolloide, p. 13. Přibram[61] has recently shown that the swelling and contraction of muscle are to be ascribed to absorption and expulsion of water from the cells of the muscle; the rapid changes taking place, are brought about by changes in the concentration and composition of the liquid components of the muscle. Lactic and phosphoric acids are produced with enormous rapidity, and of relatively high concentration; during a short period these cause instantaneous changes in the swelling of the muscle cells, followed by a return to their original state. For the evidence of this the original paper must be consulted, but it goes to show that the equilibrium of the system--colloids, electrolytes, water--depends on the proportions and quantities of each of these constituents. The phenomena we are considering thus conform to the general law of mass action (Guldberg and Waage). [61] Přibram, Die Bedeutung der Quellung und Entquellung für physiologische und pathologische Erscheinungen. Kolloid Chem. Beihefte, ii. p. 1. *Influence of Solid Matter in the Bate.*--It was first pointed out by Wood (J.S.C.I. 1899, p. 991) that a filtered puer liquor had less action on the skin than an unfiltered liquor containing much finely suspended matter. On adding an inert solid (kaolin) to the filtered bate, the action was hastened. No doubt much of the colloidal matter, and with it some of the enzymes, are removed by filtration, but nevertheless, the suspended particles in the bate have some effect on the process. In a most interesting paper, Perrin[62] has recently shown, that the granules in suspension in a colloidal liquid function like the invisible molecules of a perfect gas with a molecular weight of 3·3 x 10^{-9}. Such granules are endowed with the molecular (Brownian) movement, and therefore may exert a mechanical effect on the fibres of the skin. Although the mass of the particles is large[63] compared with molecular dimensions, they are small enough to penetrate the pores of the skin, and where puering is carried too far, may become deposited beneath the fine hyaline layer, and thus render it cloudy and unsuitable for fine colours. [62] Perrin, Kolloid Chem. Beihefte, i. p. 221.