PROGRESS IN PHYSICAL CHEMISTRY. 305 



The densities of solutions of potash and soda have been 

 determined by Pickering (144). In the case of soda the 

 curves representing at constant temperature the variation 

 of the density with the concentration show eight " breaks " 

 at points corresponding with the composition of definite 

 hydrates, six of which are known in the free state. The 

 other two "breaks" correspond with those of the freezing- 

 point curve. In the case of potash there are four "breaks," 

 one of which occurs at a concentration corresponding with 

 the composition of a known hydrate. 



The volume change attendant upon neutralisation was 

 found by Ruppin (145) to be additive in all the cases he 

 studied, if allowance were made for the dissociation of the 

 substances. 



Solubility. — Of the numerous investigations on solubility, 

 many of which refer to indifferent solvents, the most 

 important is probably that of Etard (146), whose observa- 

 tions extend to temperatures as high as 700 . The con- 

 centration of a solution he takes as the weight of salt in 

 one hundred parts by weight of solution, consequently his 

 experimental curves represent the freezing-points of mixtures 

 of substance and solvent, and the ends of the curves 

 correspond with the melting-points of the pure substance 

 and the pure solvent. In general, each curve both for water 

 and other solvents consists of several straight lines. The 

 observations refer to salts in water, mercuric chloride in 

 thirteen organic solvents and cupric chloride in nine organic 

 solvents. In the two latter cases evidence was obtained of 

 combination between substance and solvent. Determina- 

 tions were also made on sulphur in several organic solvents 

 at temperatures up to its melting-point. 



Lieben finds that the solubility of calcium, barium, and 

 strontium salts of the normal fatty acids varies regularly with 

 the replacement of H by Me. If the acids are not normal 

 the variation is complex. Winkler's work on the relation 

 between the solubility of a gas and the viscosity of its 

 solvent has been examined by Thorpe and Rodger (148). 

 They find that in the case of hydrogen, oxygen, and nitrogen, 

 the diminution in solubility produced by rise in temperature 



