SCIENCE 



[N. S. Vol. XXX. No. 757 



to be exactly true in the case of actual 

 solutions. We might therefore expect cer- 

 tain small deviations from the mass law, 

 but are in no way prepared for the start- 

 ling discrepancies which are in fact ob- 

 served. This discrepancy is sufficiently 

 marked in the case of salts of the simplest 

 type, like potassium nitrate or sodium chlo- 

 ride, but is most striking in the case of 

 some salts of a higher type. The following 

 table gives for three electrolytes the values 

 of the "mass law constant" {K) at differ- 

 ent molal concentrations {G). We see that 

 when the concentration changes one thou- 

 sand fold, K changes one hundred fold in 

 the case of potassium chloride and one mil- 

 lion fold in the case of potassium ferro- 

 eyanide! For the weak electrolyte, acetic 

 acid, it is a real constant. 



This extraordinary divergence from the 

 mass law, of which I have chosen the most 

 extreme case known in aqueous solution, is, 

 however, found to an even more startling 

 degree in the case of non-aqueous solutions. 

 To the extremely bizarre conductivity 

 curves there obtained few have had the 

 temerity to apply in full the principles 

 derived from the ionic theory. Neverthe- 

 less, we are beginning to realize that the 

 phenomena of aqueous solutions are but 

 special instances of the widely varying 

 phenomena occurring in other solvents; 

 and it seems unlikely that a satisfactory 

 understanding of the behavior of aqueous 

 solutions can come except through a care- 

 ful study of non-aqueous solutions. At 

 present our quantitative knowledge of such 

 solutions is extremely limited, and does not 



encourage the belief that we have in any 

 sense a final answer to the problem of 

 solutions. 



These, then, are some of the weak points 

 of the ionic theory as it stands to-day. If 

 the case were to rest here I am afraid it 

 would be difficult to bring in a verdict for 

 the theory of dissociation. Indeed many 

 scientists, on the basis of such evidence, 

 have decided to close the hearing and to 

 class the ionic theory with other ingenious 

 hypotheses that have failed to stand the 

 test of experience. But these men have 

 not applied the one criterion by which in 

 the end every scientific proposition must be 

 judged— the test of serviceability. After 

 all, what have we said except that the ionic 

 theory is not complete ? But perfection is 

 rare in the science of chemistry. Our sci- 

 entific theories do not, as a rule, spring 

 full-armed from the brow of their creator. 

 They are subject to slow and gradual 

 growth, and we must candidly admit that 

 the ionic theory in its growth has reached 

 the ' ' awkward age. ' ' Instead, however, of 

 judging it according to the standard of 

 perfection, let us simply ask what it has 

 accomplished, and what it may accomplish 

 in scientific service. 



When we examine a little more critically 

 the unfavorable features which we have 

 mentioned, we find that they enter chiefly 

 in the application of the theory to strong 

 electrolytes. If we consider the weak elec- 

 trolytes, like ammonia, acetic acid and most 

 of the organic substances, of which a large 

 number have been investigated, we find a 

 remarkably satisfactory state of affairs. 

 For these the mass law has generally been 

 shown to hold with remarkable accuracy. 

 Indeed it is hardly too much to say that 

 every prediction from the ionic theory has 

 been quantitatively verified for all the weak 

 electrolytes which have been carefully in- 

 vestigated. The degrees of dissociation ob- 



