142 THE FITNESS OF THE ENVIRONMENT 



If the water be pure, the concentrations of 

 hydrogen and hydroxyl ions are necessarily 

 the same, for water is electrically neutral. A 

 variety of independent methods of estimation 

 have shown that at 25° (centigrade) this 

 concentration amounts almost precisely to 

 0.0000001N, in the ordinary units. 1 This 

 corresponds to 0.0000001 gram of ionized 

 hydrogen and 0.0000017 gram of ionized 

 hydroxyl in 1000 grams of water. Further, 

 the theory of solution explains acidity in 

 water by the occurrence of hydrogen ions, 

 formed from dissolved electrolytes, in excess 

 of hydroxyl ions ; and alkalinity by a similar 

 excess of hydroxyl over hydrogen ions. Neu- 

 trality is then the condition when, as in pure 

 water, the two concentrations are equal. In 

 short, expressing the concentration of ionized 



1 Concentrations are expressed in terms of chemical 

 equivalents, gram-molecules, or moles. N (normal) is the 

 symbol for this unit. The values of the concentration of 

 ionized hydrogen at neutrality as estimated by different 

 investigators are as follows : — 



