74 PHYSICAL SCIENCE 



calcium chloride. The cryohydric point of calcium 

 chloride, an extremely soluble substance, is very 

 low, and that of a mixture of salts will be lower 

 than that of either component. This lowering 

 of the cryohydric temperature, which corresponds 

 with the lowering of the freezing-point of water 

 by the addition of salt, was observed by Buchanan 

 in experiments conducted in the Engadine. 



So far the components of the system we have 

 been considering are not miscible with each other 

 in all proportions ; only a limited amount of salt 

 can be dissolved in a given quantity of water. 

 A system not subject to any such restriction, in 

 which the phenomena are as simple as possible, 

 is found in mixtures of the metals silver and 

 copper. The equilibrium of these substances was 

 studied by Mr C. T. Heycock and the late Mr 

 F. H. Neville, who determined the melting-points, 

 or rather the points of solidification, of mixtures 

 of various proportions of the two metals. At the 

 high temperatures involved, it would, of course, 

 be impossible to use a mercury thermometer, and 

 the measurements were consequently made by 

 means of a platinum resistance thermometer, 

 with which the temperature is determined by 

 observing the electrical resistance of a coil of 

 platinum wire. The metals in the required pro- 

 portion are fused in a crucible and allowed to 

 cool. As soon as solidification sets in, the rate 

 at which the temperature falls always becomes 

 less ; and, in the case of pure metals and other 

 systems where the solid has the same composition 

 as the liquid, the temperature remains constant 

 till solidification is complete, just as the tempera- 



