=Osmosis.=—Cells of all kinds, both vegetable and animal, are limited, or surrounded by a layer of cell-substance which is firmer than, and probably different in constitution from, the substance in the interior of the cell. This outer layer is a living membrane. The nutrition and growth of the cell are dependent upon the capacity of its limiting membrane for regulating the ingress and egress of water and of substances dissolved in water. The phenomena of osmosis—that is to say, of the passage of water and of solutions through membranes—are of such high importance in relation to the life of the tissues that it may be permissible to make a further digression for the purpose of describing them (_cf._ pp. 40, 128). A very simple apparatus will suffice to exhibit a phenomenon which will give an idea of the meaning of osmosis. If the top of a glass funnel, covered with a piece of bladder, so fastened to its edge as to make it water-tight, be fixed in an inverted position in a glass vessel, the glass vessel filled with water, and the funnel filled to the same level with a solution of sugar, it will soon be evident that water is passing through the membrane into the funnel. The level of the sugar-solution will rise in the tube of the funnel. If, instead of water outside the funnel and sugar-solution inside it, a strong solution of sugar be placed in the funnel and a weaker solution outside it, water will leave the weaker for the stronger solution, and sugar the stronger solution for the weaker. If some of the solution in the funnel be removed from time to time so that the pressure in it is kept down to the same level as that outside it, water will continue to enter through the membrane and sugar to leave the contents of the funnel until the concentration of sugar is the same on the two sides. The fluids will then be of identical composition, and therefore isosmotic. In the further consideration of the phenomena of osmosis, a distinction must be made between permeable and hemipermeable membranes. Suppose in the first instance that a permeable membrane is used. Let it be so placed as to separate two watery solutions of different constitution, yet of the same osmotic pressure. By their being of the same osmotic pressure is meant that they are of the same molecular concentration. The liquid A contains certain salts in solution; but the liquid B may contain the same salts in quite different proportions. It so happens, however, that the salts are so balanced that the total tension of the salts in A is equal to the total tension of the salts in B. At first there may be some change in level in the two liquids, owing to differences in rates of diffusion through the membrane of the various salts which they contain; but after a time the levels of the two liquids will be the same. To outward appearance, nothing will have happened. Nevertheless, if the experiment has been continued for a sufficient length of time, it will be found that great changes have occurred in the constitution of the two liquids. At the commencement, although their total tensions were equal, the proportions in which the various salts were distributed in A, and therefore their partial tensions, were very different to their proportions and partial tensions in B. At the end of the experiment each of the several salts is equally divided between A and B, supposing the volume of A to equal that of B. This experiment shows that the molecules of substances in solution are free to move. They behave like gases. Gases diffuse through a membrane until their partial tensions are the same in the two spaces which the membrane separates. The æther in which physicists picture gases as dissolved offers no resistance to the migration of their molecules; neither does the solvent—water, for example—prevent the movement of salts which are distributed through it.