56 



IlEPORT — 1887. 



work is suflSciently advanced to justify the drawing of general conclusions 

 from it. 



Change of volume on the precipitation of hariiim sulphate hy various 

 sulphates. 



Two series of experiments have been completed, one with solutions 

 containing one equivalent of the salts in 50 molecules of water, the other 

 with solutions of half this strength. The results ai-e as follows : — VC. is 

 the change of volume in cubic centimetres resulting on the mixture of the 

 solution of barium chloride containing 1 gram equivalent with excess of 

 the precipitant. 



BaCl9a;Hr,0 -i- Excess. 



M. x = 100 



Li, . 



Na'j . 

 K„ 

 Cd . 

 Mg . 

 Zn . 

 Co . 

 Cu . 

 Ni . 

 Fe . 

 Mn . 



MSO.a;H,0. 



VC. 

 43-6 

 43-5 

 43-9 



40-4 

 391 

 38-6 

 38-3 

 38-1 

 37-8 



the mean difference between corresponding members of the two series 

 being 3'1, due to the different dilutions. 



It is to be noted that the first three metals yield closely agreeing 

 results, and are marked off by a gap of 3'4 — 3'5 from the others, while 

 these are all comprised within 2'8 — 2'6. It is probable that this different 

 behaviour of the magnesian sulphates is due to the presence of water of 

 constitution, cadmium forming a connecting link between this group and 

 that of the alkali metals. 



Vapour Pressure of Salt Sohitions. — The work done has been confirmed 

 to a great extent by Emden (Wied. 'Ann.' 1887, xxxi. 145), who employed 

 the barometric method of experiment. He, however, maintains the truth 

 of von Babo's law, that the vapour pressure of a solution at different 

 temperatures always bears the same proportion to that of pure water, 

 or p := AP ; where p = pressure from salt solution, P = from pure 

 water, and A = a constant. This point is a most important one, for 

 unless A varies with the temperature, there would be no reason why a 

 salt should change in solubility with rise of temperature, or at any rate 

 why the solubility of all salts should not vary equally with change of 

 temperature. 



In order to settle this question new apparatus has been devised 

 whereby water and salt solution can be compared under precisely similar 

 conditions, and it is proposed to extend the observations from ordinary 

 temperatures up to 100° C, advantage being taken of Professors Ramsay's 

 and Toung's methods of maintaining constant temperatures by means of 

 the vapour of liquids boiling under definite pressures. 



