of the anhydrous acid is increased by dilution until it has combined
rith about four times its weight of water, beyond which there is an
apparent decrease." This alteration would correspond to an alteration
at or about a percentage value of 20 per cent., and will be alluded to
more especially in the sequel.
In another passage the authors express themselves as follows : " We have little doubt of the existence of hydrates in solution, but our experiments do not give any clear evidence of them," and " though we have looked for coincidences between probable hydrates in solution and changes of curvature in our diagrams, we have not found any that we could distinctly recognise."
As to these observations the authors would venture to express the opinion that the percentage values, for which observations were made, were possibly in this particular case too few in number to enable any conclusions to be drawn definitely therefrom.
Le Blaric* gives determinations of four samples of percentages varying from 14'09 to 69'18 per cent., and arrives at the same general result as Gladstone, that the values of the refraction equivalents increase as the percentage decreases, and as the result is in accord with that deduced from an examination of other substances, it is cited as evidence in favour of the dissociation hypothesis of solution.
Lastly Pulfrich,f from an examination of certain solutions, has traced out a relation between the alteration of refractive indices and contractions on admixture, as expressed by the equation
in which N and N are the values of n - 1, D and D w those of the
densities (i) for the mixture as observed, and (ii) for the mixture if no
alteration in volume had occurred, while a is a constant.
It will thus be evident that former observers have viewed their results from the three different standpoints of (i) the coexistence of unaltered and uncombined molecules of water and acid respectively ; (ii) the dissociation to a greater or less degree of the acid molecules into their constituent ions ; and (iii) an aggregation of some kind or another of water and acid molecules, if contraction in volume is taken as a criterion of chemical combination.
It has already been shown by one of us J that the determination of refractive indices is a method as delicate as that of densities for differentiating samples of water of different degrees of salinity.
- 'Zeits. Phyaikal. Chem.,' vol. 4 (1889), p. 533.
t ' Zeits. Physikal. Chem.,' vol. 4 (1889), p. 561. I 'Roy. Soc. Proc. Edin.,' TO!. 23 (1900), p. 36.
