46 PRINCIPLES OF ANIMAL BIOLOGY 



the cell contents, and is said to be isotonic or isosmotic with the cell con- 

 tents. If a second solution is made isosmotic with the contents of the 

 cell, it is also isotonic or isosmotic with the first solution. 



Theories in explanation of osmosis, of the function of the membrane, 

 and the retarding effect of the membrane upon the passage of substances 

 in solution are numerous. None are wholly satisfactory and much needs 

 3^et to be discovered concerning various phases of osmosis. 



True Solutions and the Colloidal State. — Substances placed in 

 water or other solvents behave very differently from one another. In 

 some substances the molecules separate from each other, and in many 

 of these substances some of the molecules dissociate into ions. Such 

 mixtures are known as true solutions. In a true solution the largest 

 particles of the dissolved substance are single molecules. Most of the 

 common salts, as sodium chloride, potassium nitrate, or magnesium 

 sulphate form true solutions in water. Sometimes salts in solution form 

 chemical compounds with water, but in such cases the molecules, although 

 larger, exist in true solution. 



Other substances in water do not separate into their single molecules, 

 but into particles each of which is a physical aggregate of molecules. 

 White of egg, gelatine, agar-agar and gum arable are substances which, 

 in water, exist in such molar aggregations. While these mixtures of a 

 liquid and particles of greater than molecular size are sometimes called 

 solutions, they are not true solutions. The "dissolved" substance is 

 more properly said to be in a colloidal condition. Some substances may 

 exist either in a true solution or in the colloidal state, depending in part 

 on the solvent employed. Thus gamboge, a resinous substance, is 

 truly dissolved b}^ alcohol, but enters the colloidal condition when the 

 alcoholic solution is poured into water. 



True solutions exhibit an osmotic pressure whereas substances in a 

 colloidal state show only a very low osmotic pressure~or none at all. 

 Colloidal systems involving water as a "solvent" also differ from true 

 solutions in their affinity for water, in their electrical conductivity, and 

 in a variety of "surface phenomena." The latter phenomena depend on 

 the great amount of surface between the aggregations of molecules of 

 the contained substance and the liquid in which they are held, but can 

 not be further explained here. 



Application to Living Organisms. — Protoplasm, as pointed out in 

 the preceding chapter, is a colloidal system, and has the properties 

 belonging to such a system. Many of the fundamental characteristics 

 of living matter are due to the colloidal nature of the protoplasm, though 

 only a few of them can be referred to in this book. Absorption and 

 secretion, for example, as described in Chapter VII, are largely dependent 

 upon the colloidal nature of the protoplasm. Colloidal systems may 

 serve as semi-permeable membranes (imperfect ones, of course), and the 



