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An example of a colloid is found in the albumin of an egg, which is unable to penetrate the membrane which surrounds it. Examples of crystalloids are found in solutions of salt and sugar in water. The inability of a colloid to penetrate a membrane is due to the fact that it does not form a true solution.
All substances, however, that appear to be in solution are not able to penetrate membranes, or take part in osmosis. *Kinds of Solutions in the Body.*—The substances in solution in the body liquids are of two general kinds known as colloids and crystalloids. The crystalloids are able to pass through membranous partitions, while the colloids are not.
For instance, “while soluble crystalloids are always highly sapid, soluble colloids are singularly insipid,” as might be expected; for, as the sentient extremities of the nerves of the palate “are probably protected by a colloidal membrane,” impermeable to other colloids, a colloid, when tasted, probably never reaches those nerves.
Again, “it has been observed that vegetable gum is not digested in the stomach; the coats of that organ dialyse the soluble food, absorbing crystalloids, and rejecting all colloids.” One of the mysterious processes accompanying digestion, the secretion of free muriatic acid by the coats of the stomach, obtains a probable hypothetical explanation through the same law.
That eminent chemist was the first who drew attention to the distinction which may be made of all substances into two classes, termed by him crystalloids and colloids; or rather, of all states of matter into the crystalloid and the colloidal states, for many substances are capable of existing in either.
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