74 SOLUTIONS AND PROTOPLASM [Ch. Ill 



The work of Massart was done chiefly upon bacteria. He 

 made use of the fact demonstrated by Pfeffer (see p. 41) 

 that substances, which at a low concentration attract bacteria 

 chemotactically, at a higher concentration repel them. He 

 found that, in general, the repulsions exercised by the various 

 dissolved substances are proportional to their isotonic coef- 

 ficients, when the solutions are made up as MW solutions. 

 Thus, when a 10 MW % concentration of a substance with iso- 

 tonic coefficient 2 just begins to repel bacteria, a substance 

 which just begins to repel in a 5 MW % concentration has an 

 isotonic coefficient of 4.* 



§ 2. Effect of Varying Density upon the Structure 

 AND General Functions op Protoplasm 



Under this head we may consider, (a) the effect upon the 

 general structure of protoplasm ; (6) the modification of gen- 

 eral functions, and (c) the production of death. 



* Starting from the observations of Pfeffer and de Vries the modern school 

 of physico-chemists has greatly extended our knowledge of solutions. As a 

 result of their work it appears that the validity of de Vries' law will not hold 

 strictly for all solutions at all concentrations. For the number of efiective 

 particles in every solution of electrolytes, namely, of salts, bases, and acids, is 

 greater than the number of molecules put into the solution ; because a certain 

 proportion of the dissolved molecules break up or dissociate into their constitu- 

 ent ions, and the osmotic pressure is determined by the number of both 

 molecules and free ions in the solution. In the case of sugar, the alcohols and 

 non-electrolytes in general,* no dissociation occurs. ' In a normal solution of 

 potassic chloride, on the other hand, 76.5% of the molecules dissociate, each 

 forming two free ions. Since 24.5 % of the molecules are Intact and there are 

 151 free ions percent of the molecules introduced, the total number of mole- 

 cules and free ions in the solution is 175.5% of the molecules introduced and 

 the osmotic effect of a normal solution of KCl is 1.755 times that of a normal 

 solution of sugar. The percentage of molecules of any electrolyte, as for 

 instance KCl, which dissociate in solution increases as the strength of the 

 solution diminishes, eventually becoming 100. Thus, in one-half the normal 

 solution, 78% of the molecules of KCl dissociate ; at 0.1 times the normal solu- 

 tion, 86%; at 0.01, 94%; at 0.001, 98%. Also, the percentage of molecules 

 dissociated .in normal solutions of different electrolytes varies. Thus, in such a 

 solution of NaCl, 67.5% of the molecules are dissociated; of LiCl, 61%; of 

 CaCl2, 53% (each into 3 ions) ; of MgClj, 40%; of KI, 79%; of MgSOi, 19%; 

 of Na2S04, 35.6 % (each into 3 ions); and so on. Valuable and extensive tables 

 for the determination of the percentage of dissociation at different concentrations 

 will be found in Whetham, '95. 



