74 president's address — section b. 



and while the most of these relations do not hold good with mathe- 

 matical exactness, the relations are of very great value in the case of 

 refractive power and of absorption spectra measurements, and have 

 been of much use in fixing the constitution in certain cases. 



Again, in the domain of Chemical Mechanics, all types of reaction 

 velocity under isothermal conditions are capable of representation by 

 mathematical equations. In the simpler reactions, the velocity is 

 greatest at the start and gradually decreases as the reaction proceeds ; 

 and in the case of reactions whose progress is complicated by one of the 

 products being a catalyst, the reaction velocity has a maximum at 

 Bome other time than at the start ; in a system with several consecutive 

 reactions and with an intermediate product acting as a catalyser, 

 Lotka (1910) has shown that we have the conditions for the periodic 

 reaction. 



The agreement of experimental results v,ath the mathematical 

 equations founded on the LaM^ of Mass Action leaves nothing to be 

 desired in many cases, and our failure to have a complete correspondence 

 in every instance is due to our not appreciating all the conditions at 

 work. 



My predecessor in this chair pointed out at our last meeting how 

 very changed our ideas in regard to double decomposition and ionic 

 reactions in solution would become, if not water but some other substance 

 happened to be our " universal " solvent. 



It is important that we should never forget to look at reactions 

 in solution from that general point of view, but at the same time one 

 must be glad that most investigations on solutions have taken place on 

 aqueous solutions, as these have been very fruitful, and as they are 

 of the most direct practical application in Biology and Medicine. 



We cannot but marvel at the rapid advance of the Theory of 

 Ionic Dissociation and at the complete manner in which it is now 

 applied to water and aqueous solutions. I consider that one of the 

 best examples that could be furnished of the progress of chemical 

 mechanics is the exactitude of our present knowledge of the dissociation 

 of water. 



The dissociation constant of water was determined by Kohlrausch 

 and Heydweiller in 1894 by measurements of the electrical conductivity 

 of specially purified water. 



No matter how scientists may have admired the skill of these 

 investigators in overcoming the experimental difficulties of their 

 problem, many must have doubted whether the value obtained was not 

 too high. And yet the correctness of the value obtained then has 

 been fully borne out by determinations conducted by others. There 

 are three other distinct methods which can be used to determine the 

 dissociation constant of water and each of these three methods can 

 be applied in a variety of ways, yielding however in all cases a value 

 very close to that obtained by the conductivity method. 



