582 



Professor Sir James Deirar 



[March 



method of mixture, in which a given quantity of the substance to be 

 experimented on at a higher temperature, is mixed with a. given 

 (|uantityof a known fluid substance at a lower temperature; from the 

 iinal temperature acquired by the two, the quantity of heat given up' 

 by the former can be determined. In this method the main difficulty 

 is the isolation of the mixture from external heating or cooling 

 influences during the experiment, and its success in the hands of 

 Regnault was due to the skill with which he accomplished this. 



Evaporation is a means of absorbing heat ; no evaporation can 

 take place without absorption of heat, and usually this absorption is 

 relatively great. The well known experiment of the freezing of 

 water by its own evaporation by the methods of Leshe and Wollaston 

 are illustrative of the use of the latent heat of evaporation to produce 

 lower temperatures. Further, by a law of Dalton, evaporation takes 

 place most copiously into a space whicli is kept free from vapour of 

 the same kind as that coming off. 



It has long been known that by the passage of air through volatile 

 liquids a considerable reduction of temperature may be effected 

 depending on the particular substance selected, the isolation of the 

 liquid from external heat, and the use of air at the low temperature 

 reached in each case. The following table gives the general results of 

 the temperatures recorded by different experimenters when ether, sul- 

 phurous acid, methyl chloride, ammonia, and ethylene, were employed. 



It is interesting to notice that the limit of temperature reached 

 by this means is in each case about half the absolute critical temixira- 

 ture. Thus for ethylene the absolute temperature of evaporation is 

 '11 if — 182°, or 141°, and its absolute critical temperature is 27;>° 

 4- 10% or 28:>°, giving the ratio • 50. Now if this approximate 

 relation holds good for a substance like liquid nitrogen, then we 

 should anticipate that by passing a current of a gas througli it like 



