PROGRESS IN PHYSICAL CHEMISTRY. 291 



So/ids. — A good deal of work has been done on die fusi- 

 bility of mixtures of solids. If crystallisation be instituted 

 in a fused mixture of two isomorphous salts, the crystals 

 which begin to separate consist of both of the salts present, 

 and not of one of them only, as is the case with ordinary 

 mixtures. For this reason, on plotting the freezing-point 

 and composition of isomorphous mixtures in a co-ordinate 

 system, the curve is continuous and does not consist, as in 

 the ordinary case, of different parts which correspond with 

 the different salts which separate on freezing. In a paper 

 by Le Chatelier (75) this rule was apparently disobeyed, 

 but Ktister (76) has shown that several of the pairs of salts 

 used were not truly isomorphous, and in a later paper Le 

 Chatelier (yy) obtains curves for isomorphous carbonates 

 which conform to the rule. That this point should be 

 satisfactorily settled is important, for in observations of the 

 freezing-point, we have evidently a means of detecting 

 isomorphism. 



Observations on the melting-point may also be used to 

 detect the formation of a double compound. The freezing- 

 point curve of an ordinary binary mixture, plotted as above 

 described, shows a minimum corresponding with the eutectic 

 mixture. If, however, a double compound be formed the 

 curve will exhibit a maximum corresponding with the 

 double compound, and two minima corresponding with 

 the eutectic mixtures of the double compound with 

 each of its constituents. In this way Le Chatelier (78) 

 concludes that a double compound is formed in the case of 

 lithium and potassium carbonates. 



In a similar way from the freezing-point of triple alloys, 

 Heycock and Neville (79) infer that the compound 

 Au Cd exists in solutions where the solvent may be tin, 

 bismuth, thallium, or lead. There is also evidence that 

 Ag 2 Cd is formed in solutions in lead, tin, and thallium. 

 Gold and aluminium when dissolved in tin form the stable 

 insoluble compound Au Al 2 . Laurie (80) has also tried to 

 show the presence of compounds in alloys by observations 

 on electromotive force. His method may thus be illustrated. 

 If a plate of copper and another made partly of zinc and 



