Chemical and Physical Notes 43 



affirmed that the temperature of the thermometer is identical 

 with that of the mixture. 



The determination of the concentration of the cryohydric 

 brine produced presents no difficulty, or only such as a 

 chemist knows how to deal with, and as it is an operation 

 which cannot profitably be attempted by any except a trained 

 chemist with considerable laboratory experience, it need not be 

 further described. The chemist may, however, be reminded 

 that he has the choice of two ways of approaching the subject 

 . the analytical and the synthetical. Further, he may either 

 take his freezing mixture and allow it to melt, or he may 

 make his presumed cryohydric solution, and allow it to freeze. 



Cryohydrates of Salts forming Isomorpkous Mixtures. In 

 the study of the cryohydric constants of mixtures, a peculiar 

 interest attaches to mixtures of isomorphous salts which form 

 mixed crystals, such as the nitrates of barium, strontium and 

 lead, the nitrates of potassium and sodium. 



We have seen that in the case of salts which, although 

 they crystallise in the same system, do not form mixed 

 crystals, as for instance NaCl and KC1 or NH 4 C1, a mixture 

 of any two is more soluble than either separately, and con- 

 sequently the cryohydric temperature is lowered. It is easy 

 to imagine why this should be. Looking to the analogy 

 between salts in solution and gases, a certain mass of water 

 when saturated with, say NaCl, is still virgin with regard to 

 KC1 which dissolves in it with ease. But the introduction of 

 KC1 interferes with the free meeting of the particles of NaCl, 

 which on the slightest lowering of temperature are prepared 

 to unite and fall out as crystals. Therefore the effect of 

 introducing this indifferent body KC1 is to make the solution 

 of NaCl, which was saturated, no longer saturated : in other 

 words it depresses its temperature of saturation. But the 

 cryohydric temperature of a saline solution is the freezing 

 temperature of the saturated solution. This has been lowered : 

 therefore we see that the cryohydric temperature of a mixture 

 of NaCl and KC1 must be lower than that of NaCl alone ; and 

 it is quite independent of whether the cryohydric temperature 

 of KC1 is high or low. 



