membranes in plants and animals approach ideal semipermeability while they are living. Ideal semipermeability with respect to particular dissolved substances has been achieved and is found in living organisms.
It is to be remembered that in case of semipermeable membranes the solvent will flow from the less concentrated to the more concentrated side of the membrane, so that if we wish to extract water we need only to make the outside more concentrated than the inside. If we wish to add water, we make the outside less concentrated than the inside; that is, we use pure water outside, as has sometimes been done unfairly to swell oysters and make them appear “fat.”
It is also to be remembered that the degree of permeability of membranes does not necessarily remain unalterable. The permeability of the membrane can very readily be changed, as will be seen later. There is reason for believing, for example, that the permeability of fish to salt increases after death—for stale fish strike through more quickly than fresh fish—and that permeability increases at temperatures near the freezing point of water.
The tissues of fish consist mostly of cells. Each cell is a bag of semiliquid, like the white of egg. The surface of every cell either is or acts like a semipermeable membrane. If we surround the cell with water, the inside will be more concentrated than the outside and water will go in. If we surround the cell with strong salt solution, water will pass out to the salt. Some salt will also pass into the cell, which fact shows that the cell wall is not ideally semipermeable.
But what of the protein within the cell? Why does it not come out while the salt is going in? In order to answer these questions it is necessary to pass from a consideration of the nature of the membrane in osmosis to a consideration of the nature of the dissolved substance.
By a great many experiments it has been found that some dissolved substances never pass through membranes under any circumstances, while others will pass through some membranes. It is found that those which never pass through are also those which on drying out do not crystallize but shrink to a tough mass. They are called colloids. Examples of them are glue, albumen, gelatin, and soap. The smallest possible particle of these substances is comparatively large, too large, we may imagine, to go through the texture of the membrane. They are not only large of molecule but complex in structure. The bulk of animal bodies consists of colloids called proteins, dissolved in water. The other class of substances, those that may pass through membranes and which on drying out crystallize in regular geometrical shapes, are the crystalloids. Examples of this class are salt, sugar, and like substances. It is not to be supposed, however, that all crystalloids will pass with equal facility through any given membrane. Nearly all membranes are in some measure selective of particular crystalloids. The ideal semipermeable membrane permits none to pass, but as membranes degenerate from ideal semipermeability to complete permeability they permit more and more of these dissolved things to pass through.
The phenomena of osmosis having been briefly reviewed, one may readily perceive the importance of applying the principles to the salting of fish. Salt is brought in contact with the exterior of the
