296 SCIENCE PROGRESS. 



physical difference between the active and inactive molecules. 

 The most important of these is that due, in the first place, 

 to Arrhenius, who supposes that an electrolytically active 

 molecule is dissociated into its ions. We might, however, 

 as far as we have yet gone, imagine that the necessary 

 condition for electrolytic activity was a certain temporary 

 arrangement of the atoms in the molecule, or that two mole- 

 cules could interchange their ions only when they collided 

 in a certain definite way. Let us, then, for the present, call 

 the state of activity of a molecule ionization, and not use 

 the word dissociation, which commits us to one hypothesis. 



It is evident from what has been said, that the ionisation 

 of a solution of a certain strength [i.e., the fractional number 

 of its molecules electrolytically active at any moment) is 

 measured by the ratio of its molecular conductivity to the 

 molecular conductivity of a solution of the same substance 

 at infinite dilution, when all the molecules are active. Thus, 

 if we denote the molecular conductivity by the symbol jx, and 

 its value at infinite dilution by ;t M , we get for «, the co- 

 efficient of ionisation, the value fi/fi^. 



In the year 1883, Arrhenius published a paper 1 in which 

 he showed that this coefficient of ionisation was intimately 

 connected with the chemical activity of the solution, and 

 with the amount by which the osmotic pressure of an 

 electrolytic solution exceeds that given by the solution of a 

 non-conducting substance. The comparison has since been 

 extended by Ostwald and others, and a few of the results are 

 given below in a tabular form. 



The subject of chemical affinity has lately been examined 

 in the pages of this magazine by Dr. James Walker, and 

 the idea of a specific coefficient of affinity, expressing a 

 definite chemical property of any given solution, will be 

 familiar to its readers. 



In the following- table, the first three columns give the 

 constants of affinity of a few acids, measured in different 

 ways: (I.) by observing the volume-changes accompany- 



1 Mem. a FAcad. des Sciences de Suede, 1 883. Also B. A. Report, 

 p. 357. 1886. 



