eral substance equally soluble would preserve in the same way that salt does, but salt happens to be the only one that the human palate and stomach will tolerate.
HOW SALT EXTRACTS WATER.
At the risk of appearing verbose the writer undertakes to elucidate the principles that govern osmosis, because osmosis is nearly the whole principle of salting fish. Without a knowledge of osmosis people may salt fish successfully by rule, but without such a knowledge it is quite impossible to understand the process.
If a thin animal skin or membrane separates two liquids and if the liquids are alike and of the same concentration, nothing happens. But if they are unlike and of different concentration, one or the other or both of the liquids will pass through the skin to the other side. This passage through the skin or membrane is called osmosis. Just what components pass through the membrane, in what direction, and how much depend on many circumstances. For the purposes of salting fish water is always the liquid, plus whatever is dissolved in the water. The dividing membrane is the skin of the fish and the membranous inclosures of the microscopic cells of which the substance of the fish is composed. We thus have water and salt outside, cell membrane between, and fish juice, or protoplasm, inside, and we desire to know what will happen and how we can influence the process to suit our needs. The quantity and direction of flow through the skin or cell membrane will depend on (1) the nature of the dividing membrane, and (2) the nature and quantity of the substances dissolved in the water on each side.
The nature of the dividing membrane will be considered first. Almost any substance can be made into a thin film or membrane. Such things as glass, tin-foil, and mica may be exceedingly thin, but are totally impermeable and therefore uninteresting in the present connection. But other membranes or films, such as parchment paper, gelatin films, animal bladders, and goldbeater's skins are permeable to a greater or smaller degree. Suppose pure water were on one side of a membrane and water containing dissolved salt on the other. If the membrane is perfectly permeable to all constituents, water will pass through to the salt solution and salt will pass through to the water, and these movements will continue until the two sides are alike and then stop. It is always the tendency for the two liquids to come to equilibrium, and they would do so if the membrane were perfectly permeable. Nearly all membranes, however, permit a freer flow of the solvent, in this case water, than they do of the solute (that which is dissolved), in this case salt.
If the membrane permits the water to flow but absolutely prevents passage of a dissolved substance, the membrane is said to be semipermeable. In the example taken above, of pure water on one side and salt solution on the other, if the membrane were semipermeable then the water would pass through to the salt solution, but the salt could not get through to the water. The level of the pure water would fall and that of the salt would rise. The difference in liquid level would exert a pressure called osmotic pressure. Ideally semipermeable membranes are not realized in nature, though some of the
