72 SOLUTIONS AND PROTOPLASM [Ch. Ill 



in order that no endosmosis or exosmosis should occur through 

 the cell wall.* He next determined the same thing for some 

 other substances, e.g. KI, and found that the degree of con- 

 centration which produces no osmosis is, for two different 

 solutions, proportional to the molecular weights of the salts 

 dissolved in them. Solutions which produce the same osmotic 

 effect DE Vkies called isotonic. A solution of 0.746% KCl is 

 isotonic with a solution of 1.661% of KI, for the molecular 

 weight of KCl is 74.6, and that of KI is 166.1. Thus the first 

 result which de Veies gained was that the osmotic effect of 

 solutions of salts of similar structure depends upon the number 

 of their molecules in the solution. 



The second conclusion of db Vbies was that salts of dis- 

 similar structure have different osmotic properties, even when 

 the number of molecules in the two solutions is the same. 

 Thus, he found that with an equal number of molecules to the 

 solution (molecular -weight solutions f ) : — 



(1) All salts of alkalis with one atom of metal to the molecule are 



isotonic (formula, R'A' [composed of a monad metallic 

 radicle, B, and a monad acidic radicle, A]) ; 



(2) All organic compounds with no metal radicle have two-thirds the 



osmotic action of the first group ; e.g. cane sugar, CiaHsjOn- 1 



* As is well known, when a fully developed plant cell is put into a strong 

 saline solution the living plasma sac separates from the cell wall and contracts, 

 eventually, into a ball, — the result of the chylema flowing out of the protoplasm 

 (plasmolysis) . The weaker the concentration, the less marked the plasmolytic 

 phenomena. Finally, a concentration is reached so weak that the separation of 

 the plasma sac hardly occurs or is limited to a single corner. This concentra- 

 tion may be regarded as equal to that of the cell-sap — as that at which no 

 osmosis occurs. (See Fig. 8.) 



t I shall use the phrase " molecular- weight solution " to indicate solutions in 

 the making up of which the molecular weight of the substance in grammes, dis- 

 solved in 100 g. of water, is used as the unit of concentration. It will often be 

 convenient to abbreviate it as MW% sol. Chemists frequently use as a unit 

 solution, called "normal" solution, the molecular weight in grammes dissolved 

 in 1000 g. of water. Our MW%sol. is therefore equal to one-tenth of a "normal" 

 solution. 



X The fact that glycerine can be absorbed by some plants has introduced a 

 complexity into the determination of its isotonic coefiBcient. This determina- 

 tion has been made the subject of a special investigation by de Vries ('88), who, 

 by the use of slowly absorbing plants, has found the isotonic coefficient to be 1.78, 

 which agrees approximately with the number given above for organic compounds. 



