20 Diffusion and Osmotic Pressure 



IV. TERMINOLOGY FOR SOLUTIONS OF DIFFERING 

 CONCENTRATION 



To designate different concentrations of solutions, the 

 most common method among physiological writers has been, 

 until quite recently, that of percentage. An example of this 

 method will explain its use. A solution is said to be a 5 per 

 cent, solution of a certain solute in a certain solvent when it 

 is composed of five parts by weight of solute to ninety-five 

 parts by weight of solvent. But solutions of different solutes 

 in the same solvent depend for their physical properties upon 

 the relative number of solute particles which they contain 

 per unit volume. A glance at a table of atomic weights will 

 make it clear that any method by weights which has as its 

 basis the percentage system cannot readily be adapted to a 

 discussion of the relative number of molecules contained in 

 equal volumes of solutions of different solutes. Atomic 

 weights, and therefore molecular weights, cannot readily be 

 compared in terms of percentage. As long as physiologists 

 persist in using this antiquated method in the preparation of 

 their solutions, so long will they fail to arrive at any far- 

 reaching principles concerning the chemical and physical 

 nature of the substances used. 



A more scientific method is that based on the relative 

 number of particles of solute in unit volume of solution. 

 We cannot, of course, actually count the molecules of any 

 substance, but from a knowledge of the relative weights of 

 the molecules of different bodies it is easily possible to get 

 several masses of different substances, each of which will 

 contain approximately the same number of molecules. The 

 weights of such masses must be to each other as the molecu- 

 lar weights of the respective substances. For instance, 342 

 grams of cane sugar (mol. wt. 342) must contain the same 

 number of molecules as 180 grams of glucose (mol. wt. 180), 

 for the molecular weights give the relative weights of the 



