SALTS, ACTION OF.—The inorganic salts, as used in medicine, have effects on the body which cannot be explained by their chemical reaction with the various tissues. These changes depend on a disturbance in the relative density of the fluids of the body, and are governed by recognised physical laws. An idea of the action of these principles will explain what occurs. If two fluids, holding in solution different amounts or kinds of salts, are allowed to come in contact, they will gradually mix, forming one uniform fluid. This is called diffusion. If two such solutions are sepa rated by a permeable membrane, and one of them contains a salt, the process of diffusion will go on through the membrane, so that finally there will be an equal amount of equally salty fluid on each side of the membrane. This diffusion through a membrane is called osmose or osmosis. If the salt be of such a kind that it cannot pass through the membrane, the fluid on the other side will tend to come through to the liquid containing the salt, always tending toward a uniform solution. If the two fluids contain different amounts of different salts, some able to penetrate the membrane and some not, the result will depend on the difference in the concentration of the salts, modified by their degree of penetrating power. When that condition is reached where there is no tendency for a current to flow in either direction, the solutions are said to be isotonic.
The body is made up of innumerable little cells, each enclosed in a mem brane, and for the most part bathed in a surrounding fluid, the lymph. Under normal conditions the contents of the cells and the lymph are isotonic ; that is, balanced as far as possible in regard to the character and amount of the salts held in solution. When the lymph is changed in its composition, there results a disturbance in the balance of the lymph and the contents of the cells. Currents are, therefore, set up through the cell walls ; and an interchange of salts or fluid, as the case may be, takes place between the contents of the cells and the surrounding fluids. This illustrates the principle upon which salts affect the body from a physical standpoint. The absorption and excretion of the inorganic salts and a few other substances are partly explained by the osmotic relations between the fluid in the body and that outside of it, as the absorption of common salt from the intestinal canal through the walls of the gut. Certain cells are said to have a direct attraction for certain salts, regardless of these laws, so that their action is modified to some extent. A salt is a combination of two other substances, soda and chlorine, for instance, forming common salt. These substances act separately on the body, and are spoken of in this connection as ions, or component atoms.