274 REPORTS ON THE STATE OF SCIENCE. 



E. — Theories of the Action oj Acids in effecting Inversion. 

 E I. —The Ionic Theory/. 



The theory which has dominated the literature of the subject during 

 the last fifteen years is that proposed by Arrhenius as a corollary to his 

 ionic dissociation hypothesis. The ionic hypothesis was first advanced 

 ,^Qq in 1883, in a paper to the Swedish Academy, as an improvement 



' on the Clausius-Williamson dissociation hypothesis. Arrhenius 

 took the electrical conductivity of an acid as a measure of its chemical 

 affinity and gave a table comparing Ostwald's and Thomsen's 'avidity 

 values ' with the conductivities of a number of acids. This thesis, 

 although noticed by Ostwald {see Section D), did not at the time attract 

 universal attention. For instance. Lodge, in his report on electrolysis to 

 , r,n^ the British Association in 1885, does not mention it. In ' Zeits. 



' Physik. Chem.,' vol. xxii., Arrhenius brought his theory into popular 

 , noy notice by explaining the exceptions to Raoult's cryoscopic laws 



■ and Van't HoiF's laws of osmotic pressure of solutions (published 

 in 1886) by means of his dissociation assumptions. The 'activity co- 

 efficient,' which Lodge had called the ' dissociation ratio,' viz., the ratio 

 of electrolytically active to the total number of molecules, could be 

 ascertained, hence the 'ionisation' and the freezing-point depression 

 1 8SS could be calculated. Where the agreement was bad Arrhenius 

 showed that the freezing-point determinations, not the con- 

 ductivities, were at fault. Although Arrhenius in this paper discussed 

 the various physico-chemical properties of solutions of electrolytes, he 

 made as yet no reference to the inversion of sugar. 



, ooq In 1889 Arrhenius definitely stated that the hydrolysis of cane 



sugar is a property of the free hydrogen ions of the acid. There 

 were a large number of facts to be explained away before this theory 

 could be made plausible. He saw that ' a difliculty arises from the fact 

 that the velocity of inversion . . . can under some conditions be 

 increased . . . although the number of acting ions is decreased.' From 

 a consideration of the temperature coefficient of the change, however, 

 Arrhenius postulated his theory of an ' active part ' of the cane sugar ; 

 this part was arbitrarily assumed to change to meet the requirements of 

 his ionic theory in every case.' 



, nqf) Trevor accepted Arrhenius's theory in toto and used the inver- 



sion of sugar as a means of estimating the number of hydrogen 

 1 ng i ions in solutions of very weakly acid substances. Noyes performed 

 a similar work with the bitartrates. 

 The principal argument in favour of the above view is the result 

 loqy arrived at by Palmaer and also by W. A. Smith, namely, that in 

 1 oqo' very dilute solutions the rate of inversion is strictly proportional 

 to the number of hydrogen ions. These researches are criticised in 

 Section G 1. 



On the other hand, the variation in the activity coefficient of an acid 

 when used to hydrolyse a different sugar (Section M), the eff"ect of adding 

 a neutral salt of the acid (Section J), the influence of high pressures 

 (Section K) and of alcohol (Section H 2), coupled with the evidence 

 1 q/^/. brought forvpard by the author elsewhere, all militate against the 

 acceptance of this hypothesis. 



' See Section F. 



