Chemical and Physical Notes 39 



containing 357 grms. of NH 4 C1 to 100 grms. of H 2 O, be now 

 exposed to a temperature of I5'8C., its temperature will 

 fall gradually, until I5'8 C. is reached. During the process 

 of cooling a large amount of the chloride of ammonium 

 crystallises 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 crystallising. At I5'8 C. the saturated 

 solution of chloride of ammonium contains 22'9 grms. of the 

 salt to 100 grms. of water, so that I2'8 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 Had the solution been originally made to 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 I5'8 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 cryo- 

 hydric 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 temperature would remain 

 constant at - I5'8 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 temperature of 

 + 15 C., its temperature will first rise to I5'8 C., at which 

 point it will remain stationary while the mass liquefies. 



