80 THE ANTARCTIC MANUAL. 
water is as much dissolved in the salt as the salt is in the water. 
When the solution is sufficiently cooled, ice separates in crystals 
from the liquid salt just as it did from the gaseous air, and the 
Sreezing-point of a solution is in reality a dew-point. The snow de- 
rived from these different sources has identical properties ; and when 
real snow of atmospheric origin is mixed with a saline solution, it is 
as impossible to free it from salt and to get it to melt at 0° C. as it is 
to do so with the crystals formed by freezing a saline solution. 
This matter has been dealt with at some length, not only because 
it is of importance for the Antarctic explorer, but because it is a 
matter of the highest importance in chemistry. The whole of that 
great branch of physical chemistry, which has the distinctive title 
of cryoscopy, depends on the fact that a saline solution, in freezing, 
yields pure ice; yet, in treatises on the subject, no adequate proof of 
this fundamental fact is offered. 
Cryohydrates.—When a saline solution has been exposed to con- 
tinued freezing, it finally acquires a concentration at which any further 
removal of water in the form of ice causes the precipitation of salt, 
because at the temperature attained the solution is saturated with the 
salt. If the cooling is continued the temperature remains constant, 
while ice and crystals of the salt separate out pari passu, and finally the 
whole solution may solidify to a porcelain-like mass, which has been 
called the cryohydrate of the particular salt. If warmed it will melt 
again at a constant temperature until all the salt has been dissolved, 
when the temperature will begin to rise. The case is quite analogous 
to the boiling mixtures* of constant temperature produced by 
blowing steam through salt, the temperature remains quite constant 
at that of the boiling saturated solution until nearly all the salt is 
dissolved, after which the temperature falls in proportion as the 
solution is diluted by the condensation of steam. 
The temperature at which the ecryohydrate forms has been called 
the cryohydric temperature. At this temperature crystals of ice 
and of the salt remain side by side without melting each other, and 
they behave with the same indifference to each other at lower tem- 
peratures. At temperatures above the cryohydric point they cannot 
be lrought together without melting, when they produce the eryo- 
hydric temperature. The cryohydric temperatures of different salts 
differ. krom the observations of Weyprecht, Nordenskjéld, and 
others, we learn that even at a temperature of — 40° C. there is liquid 
brine in the surface layers of the sea-ice ; so that the eryohydric point 
* ‘On Steam and Brines,” by J. Y. Buchanan, F.R.S. Trans. RSE. (1899), vol. 
EXXIX. p. 529. 
