Page:Text-book of Electrochemistry.djvu/76

 disturb its simplicity. This recalls the behaviour of gases which at high pressure deviate from Boyle's law.

It is well known that van der Waals has sought to explain these deviations by forces of attraction which act between the gas molecules. In the same way we may assume that in solution there is an attraction between the dissolved molecules, and also between these and the molecules of the solvent. The former attraction causes a diminution in the molecular lowering of the freezing point as the concentration increases^ and the latter causes a rise. Both cases occur frequently, the latter particularly in aqueous solutions, and the former in most other solutions. Almost the greatest deviation which has been noticed at high concentration was with a solution of cane sugar in water. In this case the molecular depression rises (almost proportionally with the concentration) from the value 18*6 at high dilution to 27*0 for a normal solution. Consequently, when the molecular weight of a dissolved substance is to be determined, it should be investigated in very dilute solution, or it should be examined at several concentrations, and from the results the value at concentration is ascertained by extrapolation. Baoult found this nile erapiricaUy.

Alloys. — W. Eamsay {11) investigated the vapour pressure of solutions of various metals in mercury in the following way. A U-tube, closed at one end, was filled with mercury, and a similar tube contained the amalgam to be investigated. These were immersed in a mercury bath at high temperature and the vapour pressures were measured. A lowering of the vapour pressure was always observed on dissolving foreign metals in the mercury, and Bamsay was thus able to determine the molecular weight of the dissolved metal ; for most metals, namely, Li, Na, Mg, Zn, Cd, Ga, Tl, Sn, Pb, Mn, Ag, and Au, he obtained results which agreed with the atomic weights within the experimental error. For potassium, calcium, and barium he foimd numbers which are very appreciably lower (about half) than the atomic weights, a peculiar phenomenon which has not yet been explained. The

�� �