334? DISCOURSE ON THE STUDY 



doubt the original mode of disturbance of electrical 

 equilibrium, by which electricity is excited in the 

 first instance, either by friction or by any other of 

 those causes which have been ascertained to pro- 

 duce such an effect: analogies, it is true, are not 

 wanting*; but it must be allowed that hitherto 



* We will mention one which we do not remember to have 

 seen noticed elsewhere in the case of a disturbance of the equi- 

 librium of heat produced by means purely mechanical, and by 

 a process depending entirely on a certain order and sequence 

 of events, and the operation of known causes. Suppose a quan- 

 tity of air enclosed in a metallic reservoir, of some good con- 

 ductor of heat, and suddenly compressed by a piston. After 

 giving time for the heat developed by the condensation to be 

 communicated from the air to the metal which will be thereby 

 more or less raised in temperature above the surrounding atmo- 

 sphere, let the piston be suddenly retracted and the air restored 

 to its original volume in an instant. The whole apparatus is 

 now precisely in its initial situation, as to the disposition of its 

 material parts, and the whole quantity of heat it contains re- 

 mains unchanged. But it is evident that the distribution of 

 this heat within it is now very different from what it was before ; 

 for the air in its sudden expansion cannot re-absorb in an in- 

 stant of time all the heat it had parted with to the metal : it 

 will, therefore, have a temperature below that of the general 

 atmosphere, while the metal yet retains one above it. Thus, 

 a subversion of the equilibrium of temperature has been bond, 

 fide effected. Heat has been driven from the air into the 

 metal, while every thing else remains unchanged. 



We have here a means by which, it is evident, heat may be 

 obtained, to any extent, from the air, without fuel. For if, in 

 place of withdrawing the piston and letting the same air ex- 

 pand, within the reservoir, it be allowed to escape so suddenly 

 as not to re-absorb the heat given off, and fresh air be then ad- 

 mitted and the process repeated, any quantity of air may thus 

 be drained of its heat. 



