Neither the chemical formula nor the molecular weight of collagen is known with certainty, but, from a series of ultimate analyses, the change of collagen into gelatin is represented by Hofmeister[45] by the following equations:-- [45] Allen, Comm. Org. Analysis, iv. p. 459. C_{102}H_{149}N_{31}O_{38} + H_{2}O = C_{102}H_{151}N_{31}O_{39} collagen gelatin from which it will be seen that he assigns a molecular weight of 2416 to collagen, and 2434 to gelatin. Other considerations, however, make it probable that the molecular weight is double the above figures. Gelatin is a typical colloid, and we may consider that skin is practically a colloid with a structure (a very important point, as we shall see later), but behaving in many ways as gelatin. The puer solution is to a large extent colloidal. Therefore, both the skin and the puer are in the colloidal state. In the typical crystalloid solution of an electrolyte, the dissolved body is separated into its molecules, and to a large extent into individual ions, while, in the colloid state, the units of distribution are either large and often conjugated molecules, or more frequently minute particles composed of many molecules united by cohesive attraction.[46] In the case of the skin the molecules are not free to move, but are held in place by the structure of the skin, and the fibres thus act as semi-permeable membranes, with capillary spaces between them, in which water and other fluids are merely held by capillary attraction. [46] Procter, Colloidal Chemistry. Brit. Assoc. Rep. 1908. If filtered puer liquor is put into a vessel closed by a membrane of skin, and the whole immersed in clear lime water, the puer solution becomes turbid in a short time, but the outer solution remains clear, showing that the skin is permeable to the lime solution (crystalloid), but not to the puer solution (colloid). It was observed, however, that the acids contained in the puer diffused through the membrane; this was shown by the addition of a few drops of phenolphthalein to the lime solution, when the pink colour disappeared after a short time in the neighbourhood of the skin. These phenomena are due to osmosis. It is the fundamental property of all animal membranes, to allow some substances to pass through them more easily than others. In many cases, such membranes, while freely permeable to water, are practically impermeable to certain substances in solution, and play the part of sieves in directing and controlling the diffusion. In the case of skin the phenomena are complicated by the fact that the skin combines chemically with many substances in solution, and thus we do not always know what part to assign to chemical combination and what to the osmotic phenomena.