Some Physical Properties of Nitric Acid Solutions. 107 



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. 



