SECTION III., 1900 [ 75 ] Trans. R. S. C. 
Vil.—Wote on the effect of a change in Dissociation on the Density- 
Curve of a Hydrated Electrolyte in Aqueous Solutions of different 
Concentration. 
By Howarp T. Barnes, M.A.Sc., D.Sc. 
(Communicated by Prof. John Cox and read May 29, 1900.) 
It has been possible to show in another placet from the density 
determinations of Mr. A. P. Scott and the author, that the curve 
representing the change in density with concentration for many hy- 
drated electrolytes is discontinuous. Notably this is seen for Mg CL, 
NaCl, K, SO, Na, SO,, Mg SO,, Cd SO,, Zn SO, and Zn (NO;),. 
By carefully comparing the density curves for these same salts given 
by other observers with our work, and reducing them according to the 
same plan, a close correspondence is seen to exist. 
The idea of a discontinuous density-curve has been already ad- 
vanced by Pickering”, but based on exceedingly artificial and unreliable 
calculations, so much so that the idea has been very vigorously attacked 
by Nernst. The main point of Nernst’s contention being that no such 
discontinuity has been shown in the various other physical properties of 
solutions. I desire to point out a: this time that the measurements of 
Kohlrausch on the change in the electrical conductivity of sulphuric acid 
with concentration reveals a very similar kind of change in the curve to 
that which is noted in the curves of density and concentration. 
There seems to be little reason why a change in the degree of dis- 
sociation of a hydrated salt should not be shown in the volume of its 
solution. There will be, of course, no abrupt break in the curve, but 
the complete density curve for a substance like sulphuric acid for in- 
stance may be made up of a number of different branches representing 
the degrees of dissociation of the various hydrates. The meeting points 
of these branches will be, of course, blended and rounded off, and will 
occur at the concentration where the change in dissociation of the salt 
and hydrate takes place. 
Quite recently Bancroft* has brought forward theoretical conjec- 
tures as to the effect of the solvent on certain salts in solution and comes 

14 Jour. Phys. Chem., 2, 536, 1898. 
2 Zeit. phys. Chem., 6, 10, 1890. 
For the suggestion that this may possibly be the explanation of the discon- 
tinuity in the density curves I am indebted to Prof. Bancroft. 
4 Jour. Phys. Chem., 3, 551, 1899. 
