26 INTRODUCTION TO GENERAL PHYSIOLOGY 



The number of water molecules attached varies with the different 

 ions. This fact is of importance in connection with the permeability 

 of the cell membrane to them, since inorganic ions become larger 

 than would be expected. 



In addition, however, to this cause of difference in conductivity 

 of solutions, there is another in the fact that different substances 

 are split up in very different degrees when dissolved in water. So 

 that, even if their ions move at the same rate, there are fewer of 

 them in the one case than in the other. It must always be kept 

 in mind that those molecules which are not split up into ions take 

 no part in the carriage of electrical currents. 



It is not to be understood that all organic compounds are 

 similar to sugar in being non-conductors. Some of them are acids, 

 some are bases, and some are salts. But since they are, as a rule, 

 large and complex molecules as compared with inorganic com- 

 pounds, they are not such good conductors, although many of them 

 are better conductors than might have been supposed from the 

 dimensions of their molecules. Thus, solutions of congo red are 

 very good conductors, although it is a salt of an organic acid of 

 very large molecular dimensions with sodium. 



Strength of Acids and Bases. It is well known that some 

 acids are very much more powerful chemical reagents than others. 

 Thus, hydrochloric acid in dilute solution dissolves zinc with great 

 rapidity, whereas acetic acid in the same molecular concentration 

 has very little action upon it. Now, if we compare strong acids 

 with weak acids as regards their electrical conductivity, we find 

 that the former are much better conductors than the latter. This 

 might be due either to their being more dissociated, or to the rate 

 of migration of their ions being greater. We can decide this 

 question by diluting (E., p. 175). Suppose that we take hydro- 

 chloric acid and acetic acid, each in one-tenth molar concentration. 

 The former is a much better conductor than the latter. Next, 

 dilute each to ten times its volume. We find that the conductivity 

 of the hydrochloric acid is reduced almost exactly to one-tenth. 

 This means that practically no further dissociation has occurred ; or, 

 in other words, that it was at first almost completely dissociated. 

 On the other hand, the conductivity of the acetic acid is much 

 greater than one-tenth, hence it must have become more dissociated, 

 since the original ions would only account for a diminution to one- 

 tenth. By further dilution, we can make the conductivities of the 

 two acids approach one another nearer and nearer. 



It is clear that these considerations suggest to us a method of 

 expressing the "acidity" of a solution in a numerical manner, a 

 fact of great convenience and importance. We have merely to 

 give the molecular concentration in ions ; and, since it is only the 



