SOURCES OF HEAT AND COLD. 135 



With a rapid exhaustion by the air pump, mercury in a thermome- 

 ter ball, if the ball be wrapped in flannel or fleecy hosiery and dipped 

 n ether ior sulphuret of carbon, will be frozen in two or three minutes. 



Evaporation is very extensive in its natural operation, and its uni- 

 versal prevalence is one of the great causes which prevents the accu- 

 mulation of heat on our globe, and which therefore tends very much 

 to preserve the equilibrium of its temperature. It is also occasion- 

 ally of use in the operations of art, and is sometimes employed as we 

 have already seen, to depress the temperature of particular bodies. 



2. Rarefaction. This is intimately connected with evaporation, 

 and depends upon the same principle. As condensation produces 

 heat, so rarefaction generates cold. It is seen chiefly in the aeriform 

 fluids. The remarkable example at the fountain of Hiero, has been 

 already mentioned. In air pump experiments, the thermometer falls 

 several degrees, and Dr. Darwin observed, "that if, in the stream 

 of air issuing from the receiver of an air gun, in which it had been 

 compressed, a thermometer were placed, it sunk from 5 to 7." 



In the first instance, it produces heat by its condensation, and in- 

 stantly after, cold by its rarefaction. 



Air, condensed into a reservoir and suddenly liberated from an or- 

 ifice, produces a considerable degree of cold : Gay Lussac found it 

 equal to 50 of Fahr.* 



If heat must be absorbed in evaporation or gazification, in order to 

 produce an aeriform body, more heat is required to enlarge its bulk 

 after it is produced, and, as its particles are repulsive, when the pres- 

 sure which retains them within a certain distance is diminished, the 

 particles recede and caloric is absorbed, for, otherwise their repellent 

 power could not be maintained at increasing distances, and they would 

 again approach ; when they are forcibly brought together anew by 

 compression, the heat is again given out. 



3. Chemical action. Cold is produced during the chemical ac- 

 tion of those substances whose capacity is by the union enlarged, 

 and which therefore absorb caloric. The immediate effect of chem- 

 ical union is a mutual penetration of particles, and therefore an in- 

 crease of specific gravity, and of course an emergence of heat ; but 

 it often happens also that there is an enlargement of capacity and the 

 absorption of heat which follows from this cause, is frequently suffi- 

 cient to generate a considerable degree of cold. Sulphuric acid and 

 snow afford us an illustration of both these remarks ; when first min- 

 gled they produce heat for an instant, owing to the energy of their 

 combination, but immediately after, cold is produced because water 

 is of the capacity of ten for caloric, while ice is only nine. 



* Probably from the medium of temperature. " The cold will, however, depend 

 on the previous condensation of the air." Dr. Torrey informs me that he makes 

 this experiment with Newman's blowpipe, and that, with an air thermometer, the 

 e:ffect can be witnessed at a considerable distance. 



