42 Chemical and Physical Notes 



salt and ordinary moist snow, the salt forms little local 

 freezing mixtures where it comes into actual contact with 

 a snow-crystal, and the cold produced freezes the moisture 

 adhering to contiguous snow-crystals, with the result that 

 an altogether impracticable and heterogeneous mass of lumps 

 of ice and masses of salt is formed. In the Antarctic regions 

 cold powdery snow will be common enough, and exact 

 experiments on the temperature and concentration of the 

 freezing mixtures which it makes with different salts, and 

 especially with definite mixtures of salts, will be easy, and 

 cannot fail to be interesting. 



The salts must be pure, dry, and in fine powder. When 

 mixtures are used they should be in simple molecular pro- 

 portions, as for instance, NaCl + NH 4 Cl, NaCl + 2NH 4 Cl, 

 3NaCl + 2NH 4 Cl, and so on. The weighed quantities of the 

 two salts must be thoroughly mixed in a mortar before they 

 are brought together with the snow. The experimenter must 

 feel assured that there is no risk of his freezing mixture con- 

 sisting of an indefinite association of local freezing mixtures, 

 of, for instance, snow and NaCl and of snow and NH 4 C1, but 

 that it is certainly a homogeneous mixture, every element of 

 which consists of three bodies. 



The dry snow has necessarily a low temperature. The 

 salt, or mixture of salts, when finely pulverised and intimately 

 mixed, should be cooled in a stoppered bottle to a tempera- 

 ture below o C. When the cold dry salt is mixed with the 

 cold dry snow, and the temperature, although low. is still 

 above the cryohydric temperature of the mixture, and the 

 temperature of the air when the mixture is made is also 

 above the cryohydric temperature, the temperature of the 

 mixture falls smartly to and stops abruptly at the cryohydric 

 temperature. Even when exposed to the ordinary tempera- 

 ture of an inhabited room, such a mixture, if prepared with 

 any care, will maintain a perfectly constant temperature for 

 a length of time depending on the mass of the mixture made. 

 The determination of the cryohydric temperature is thus 

 quite simple, and its exactness depends on that of the 

 thermometer, and on the certainty with which it may be 



