260 PHYSIOLOGY OF ALIMENTATION. 



sideration is a so-called non-electrolyte, that is, a substance 

 which in solution in water does not conduct the electric cur- 

 rent. Into this group of non-electrolytes belong, for ex- 

 ample, the various sugars, glycerine, and urea. Under the 

 electrolytes are classed all those substances which when dis- 

 solved in water conduct the electric current. This group is 

 composed of the acids, bases, and salts, the more typical 

 examples being the strong acids, basjes, and salts, such as the 

 mineral acids, the caustic alkalies, and the salts formed by 

 the chemical union of these two. 



VAN'T HOFF 's laws do not hold in this unmodified way for 

 the electrolytes. Solutions of electrolytes all behave as 

 though they contained a larger number of molecules than is 

 indicated by the weight of the substance dissolved in the 

 water. This apparent exception to the laws of VAN'T HOFF 

 has been explained by ARRHENIUS, who has shown that when 

 electrolytes are dissolved in water the molecules break up 

 into smaller electrically charged atoms or groups of atoms 

 called ions. This theory of ions is also known as the theory 

 of electrolytic dissociation. The degree of dissociation or 

 ionization which the molecules of an electrolyte undergo 

 varies not only with the nature of the electrolyte itself, but 

 also with a number of external conditions such as tempera- 

 ture, concentration, etc. But the amount of this dissocia- 

 tion can be determined experimentally, and when it is taken 

 into consideration the laws of VAN'T HOFF are valid for solu- 

 tions of electrolytes also. Modified in this way the first law of 

 VAN'T HOFF reads: the osmotic pressure of a solution is pro- 

 portional. to the number of molecules plus ions present in the 

 unit volume of solvent. The word particles was used above 

 to cover this conception of molecules plus ions. 



What has been said will serve to indicate the important 

 role which diffusion must play in determining the migration 

 of dissolved substances from regions of higher concentration 

 to those of lower. Because of diffusion the sugars, the salts, 

 and the digestion products of the proteins and fats enter the 



