RESPIRATION 177 



The vapor pressures, boiling points, and freezing points of 

 sugar solutions show a similar agreement between observations 

 and the new theory, as pointed out in detail in my paper. 



To physiologists the main advantage of the new theory is that, 

 as will be pointed out in detail in later chapters, it enables us to 

 utilize the kinetic theory of matter in unifying our conceptions 

 of a great number of physiological phenomena. 



The osmotic pressures observed by Pfeffer and others for dilute 

 salt solutions were far greater than corresponded to van't Hoff's 

 theory. This became quite intelligible when Arrhenius pointed 

 out in I88; 11 that the discrepancy could be cleared up on the as- 

 sumption that solutions of electrolytes are ionized to a greater or 

 less extent. Their osmotic pressures are not merely due to the 

 concentration (or, in terms of the new theory just referred to, the 

 diffusion pressure) of complete molecules of the solute, but also 

 to the concentrations of the ions present, as indicated by the vary- 

 ing electrical conductivities of different strengths of the solutions. 

 This explanation of Arrhenius was received at first with some 

 incredulity, but is now universally accepted, as the evidence in 

 favor of it is overwhelming. A dilute solution of sodium chloride, 

 for instance, is not now regarded as a solution of NaCl molecules, 

 but, practically speaking, of sodium and chlorine ions. Similarly 

 a dilute solution of hydrochloric acid is a solution of hydrogen and 

 chlorine ions. 



lonization may be regarded as a tearing apart of the molecules 

 of the electrolyte in solution on account of the molecular affinity 

 of H 2 O molecules for the atoms of the electrolyte molecules ; and 

 in accordance with this conception the ions are not stray atoms or 

 other fragments of molecules, but molecular compounds with 

 molecules of water. In pure water itself the molecules are also to 

 a certain extent ionized, as indicated by, among other things, the 

 conductivity of pure water. The products of this ionization are 

 hydrogen and hydroxyl (HO) ions, combined with molecules of 

 water. 



The acidity of a solution is due to preponderance of hydrogen 

 ions, and the alkalinity to preponderance of hydroxyl ions; and 

 when the concentrations of hydrogen and hydroxyl ions are equal 

 the solution is neutral. As, however, hydrogen and hydroxyl ions 

 are constantly reacting with one another according to the equation 



H + HO * H 2 0, 

 the product of the concentrations of hydrogen and hydroxyl ions 



11 Arrhenius, Zeitschr. f. -phystk. Chemie, I, p. 631, 1887. 



