86 THE ANTARCTIC MANUAL. 
ture, 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 eryohydric 
temperature. Even when exposed to the ordinary temperature 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 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 at- 
tempted by any except a trained chemist with considerable labora- 
tory 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 Isomorphous Miatures—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 KCl or NH,Cl, a mixture of any two is more 
soluble than either separately, and consequently 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 KCl which dissolves in it with ease. But the introduction 
of KCl, 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 KCl is to make the solution of NaCl, which was 
saturated, no longer saturated: in other words it depresses its tem- 
perature of saturation. But the cryohydric temperature of a saline 
solution is the treezing temperature of the saturated solution. This 
