318 DISCOURSE ON THE STUDY 



a single exception) to be universally abandoned. 

 The sensation of cold is as easily explicable by the 

 passage of heat outwards through the surface of the 

 body as that of heat by its ingress from without ; 

 and the experiments cited in proof of a radiation of 

 cold are all perfectly explained by Prevost's theory 

 of reciprocal interchange. It is remarkable, how- 

 ever, how very limited our means of producing in- 

 tense cold are, compared with those we possess of 

 effecting the accumulation of heat in bodies. This 

 is one of the strongest arguments adducible in favour 

 of the doctrines of those who maintain the possibility 

 of exhausting the heat of a body altogether, and 

 leaving it in a state absolutely devoid of it. But we 

 ought to consider, that the known methods of gene- 

 rating heat chiefly turn on the production of che- 

 mical combinations : we may easily conceive, there- 

 fore, that, to obtain equally powerful corresponding 

 frigorific effects, we ought to possess the means of 

 effecting a disunion equally extensive and rapid be- 

 tween such elements, actually combined, as have 

 already produced heat by their union. This, how- 

 ever, we can only accomplish by engaging them in 

 combinations still more energetic, that is to say, in 

 which we may reasonably expect more heat to be 

 produced by the new combination than would be 

 destroyed or abstracted by the proposed decomposi- 

 tion. Chemistry, however, (unaided by electric 

 agency,) affords no means of suddenly breaking the 

 union of two elements, and presenting both in an un- 

 combined state. A certain analogy to such disunion, 

 however, and its consequences, may be traced in the 

 sudden expansion of condensed gases from a liquid 



