CHEMICAL AND PHYSICAL NOTES. 83 
fall gradually, until —15°°8 C. is reached. During the process 
of cooling a large amount of the chloride of ammonium crystal- 
lises out. It is well known—and the fact can be verified in a 
moment by making the experiment—that when crystals of chloride 
of ammonium are mixed with water of ordinary temperature, 
cold is produced. The cold experienced is the measure of the heat 
absorbed in the liquefaction of the salt by solution. When the 
process is reversed and the salt is deposited from the solution this 
heat is restored, and it is appropriated in the first instance by the 
solution which is cooling and crystallising, and pro tanto it diminishes 
the rate of cooling and of ecrystallising. At —15°°8 C. the saturated 
solution of chloride of ammonium contains 22:9 grms. of the salt to 
100 grms. of water, so that 12°8 grms. of the salt have crystallised 
out. Had the solution been originally made to contain only 
22-9 grms. of chloride of ammonium to 100 grms. of water at 
+15° C., and it had then been exposed to the low temperature of 
—15°-8 C.,, the time in which the solution would have fallen to this 
temperature would have been much shorter, but the final result 
would have been the same—we should have an aqueous solution of 
chloride of ammonium of the cryohydric concentration and at the 
cryohydric temperature. If this solution were then exposed to a 
lower temperature, say —20° C., it would lose heat, but its tempera- 
ture would remain constant at —15°°8 C., and the equivalent of the 
heat removed would be apparent in the ice and salt which would 
separate out part passu so long as heat was being removed and there 
remained anything liquid from which to remove it. So soon as the 
liquid has disappeared and the mass has become solid throughout, 
the further removal of heat is represented by a fall of temperature of 
the solid mass; the temperature will fall in time to —20° C., and to 
any lower temperature to which the solid may be exposed. If it is 
then warmed by being placed, for instance, in a room having a tem- 
perature of +15° C., its temperature will first rise to —15°°8 C., at 
which point it will remain stationary while the mass liquefies. When 
the mass is all liquid the temperature of the liquid will rise rapidly 
until it finally assumes that of the room. 
We see, then, that at temperatures below —15°°8 C. chloride of 
ammonium and ice are indifferent to each other; above this tempera- 
ture they melt each other, and at temperatures between 0° C. and 
—15°°8 C. chloride of ammonium is a deliquescent salt. 
It might be thought that,.as the cryohydric temperature of 
chloride of sodium is —21°-85 C. and that of chloride of ammonium 
is —15°°8 C., the cryohydric temperature of a mixture of them in 
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