82 THE ANTARCTIC MANUAL. 
remain stationary while crystals form in the solution. This crys- 
tallisation will proceed until the solution has become a white solid 
enamel-like mass. When it has completely solidified, its temperature 
will begin to fall again, and will continue to fall until it has reached 
the temperature of the air, — 25° C. or —30° C., as the case may be. 
The white enamel-like mass is what was called by Guthrie the 
cryohydrate of chloride of sodium. Although it is no longer believed 
to be a definite hydrate in the chemical sense of the term, still it is a 
mixture of NaCl and H,O in a definite and constant proportion with 
a constant melting-point, and simulates a chemical compound so 
successfully that it is well entitled to retain its name of cryo- 
hydrate, all the more as it is convenient to have a special name to 
designate these mixtures. The cryohydrate of chloride of sodium is 
an intimate mixture of 29°6 grms. of chloride of sodium with 
100 grms. of ice. It melts at the constant temperature —21°-85 C., 
and when melted it will, if cooled, solidify again at the same 
temperature. Below this temperature chloride of sodium and ice are 
indifferent to each other; above this temperature they melt each 
other, and between this temperature and 0° C. chloride of sodium is 
a deliquescent salt. 
Again, we see from de Coppet’s table that the cryohydrate of 
chloride of ammonium (NH,CI) consists of 22:9 grms. of the salt and 
100 grms. of ice, and this mixture can exist either in the liquid or 
the solid state at -15°°8 C. In the case of chloride of sodium it was 
indicated that by making a solution saturated at ordinary tempera- 
tures (+15° C.) we obtained a solution of the concentration of the 
cryohydrate, which is a solution saturated at the cryohydric tempera- 
ture (—21°85° C.). It is a remarkable property of common salt 
that its solubility in water is almost entirely unaffected by change of 
temperature. The law holds almost universally that salts are more 
soluble in warm than in cold water. This is the case with chloride 
of ammonium. At a barometric pressure of 742 mm., when steam 
is blown through the salt it forms a boiling mixture of salt and 
saturated solution having the constant temperature of 113°°8 C., the 
temperature of boiling distilled water under the same pressure being 
99°33 C. This boiling saturated solution contains 78°7 grms. of 
chloride of ammonium to 100 grms. of water, and on cooling to the 
ordinary temperature of 15° C. the amount of the salt remaining in 
solution is only 35°7 grms. to 100 grms. of water, the difference, 
43 grms., has separated out as crystals. If the solution, saturated at 
+15° C. and containing 35°7 grms. of NH,Cl to 100 grms. of H,0, 
be now exposed to a temperature of —15°°8 C., its temperature will 
