OSMOSIS AND DIALYSIS 137 



ordinary leakage from the chimney, for while such explana- 

 tion might account for the similarity in color of the two 

 liquids, it would not account for the rise in liquid in the 

 chimney. What has happened is as follows: 



In some way, the soluble coloring matter in the chimney 

 has passed through the membrane into the jar of clear 

 water, while some of the water in the jar has passed through 

 the membrane and into the chimney as evidenced by the 

 higher level in the chimney. This tendency of two liquids 

 to mix by passage through a separating membrane is called 

 osmosis and the experiment just described is an example 

 of osmosis. 



If we continue the experiment and substitute for the 

 potassiimi bichromate solution other solutions, such as 

 grape sugar, white of egg, starch paste, peptone, etc., we 

 shall obtain varying results. When we test the water in 

 the jar we find that while grape sugar, peptone, and salt 

 solutions pass readily through the membrane into the jar, 

 white of egg and starch paste do not pass through it at all. 

 In other words, peptone, grape sugar, and salt solutions 

 are substances which mix with water by osmosis, while 

 starch paste and albumin do not. Soluble substances 

 which mix by osmosis are called crystalloids; those which 

 do not are called colloids. Grape sugar, salt solution, 

 potassium bichromate solution, and peptone are therefore 

 crystalloids; starch paste and albumin are colloids. Fur- 

 thermore, observations of the height to which the water 

 rises in the chimney in each of the above cases shows that 

 the rate of exchange is unequal, the water in each case pass- 

 ing through more rapidly than the crystalloid solution, 

 and experiment has shown that this rate of passage is deter- 

 mined by the character of the crystalloid employed. This 



