*Conditions under which Osmosis occurs.*—Osmosis may be shown by suitable experiments (see Practical Work) to take place under the following conditions: 1. The liquids on the two sides of the partition must be _unlike_ either in density or in composition. Since the effect of the movement is to reduce the liquids to the same condition, _a difference in density causes the flow to be greater from the less dense toward the denser liquid_, than in the opposite direction; while _a difference in composition causes the substances in solution to move from the place of greater abundance toward places of less abundance_. 2. The liquids must be capable of wetting, or penetrating, the partition. If but one of the liquids penetrates the partition, the flow will be in but one direction. 3. The liquids on the two sides of the partition must readily mix with each other. *Osmosis at the Cells.*—In the body osmosis takes place between the blood and the lymph and between the lymph and the cells, the movements being through the capillary walls and the membranes inclosing the cells (Fig. 27). Oxygen and food materials, which are found in great abundance in the blood, are less abundant in the lymph and still less abundant in the cells. According to the principle of osmosis, the main flow of oxygen and food is from the capillaries toward the cells. On the other hand, the wastes are most abundant in the cells where they are formed, less abundant in the lymph, and least abundant in the blood. Hence the wastes flow from the cells toward the capillaries. *Solutions.*—Neither the blood plasma nor the lymph, as already shown, are simple liquids; but they consist of water and different substances dissolved in the water. They belong to a class of substances called _solutions_. The chief point of interest about substances in solution is that they are very finely divided and that their little particles are free to move about in the liquid that contains them. Both the motion and the finely divided condition of the dissolved substances are necessary to the process of osmosis. All substances, however, that appear to be in solution are not able to penetrate membranes, or take part in osmosis.