ON THE CONSERVATION OF FORCE. 349 



cooling. If, on the contrary, the gas is suddenly allowed 

 to issue into a perfectly exhausted space where it finds no 

 resistance, it does not become cool as Joule has shown ; 

 or if individual parts of it become cool, others become 

 warm ; and, after the temperature has become equalised, 

 this is exactly as much as before the sudden expansion of 

 the gaseous mass. 



How much heat the various gases disengage when they 

 are compressed, and how much work is necessary for their 

 compression ; or, conversely, how much heat disappears 

 when they expand under a pressure equal to their own 

 counterpressure, and how much work they thereby effect in 

 overcoming this counterpressure, was partly known from 

 the older physical experiments, and has partly been de- 

 termined by the recent experiments of Eegnault by 

 extremely perfect methods. Calculations with the best 

 data of this kind give us the value of the thermal equiva- 

 lent from experiments : 



With atmospheric air 426-0 metres. 



oxygen . . . . . . 4257 



,, nitrogen . . . . . 431*3 



hydrogen ... . 4 25 '3 



Comparing these numbers with those which determine 

 the equivalence of heat and mechanical work in friction, 

 as close an agreement is seen as can at all be expected 

 from numbers which have been obtained by such varied 

 investigations of different observers. 



Thus then : a certain quantity of heat may be changed 

 into a definite quantity of work ; this quantity of work can 

 also be retransformed into heat, and, indeed, into exactly 

 the same quantity of heat as that from which it origi- 

 nated ; in a mechanical point of view, they are exactly 

 equivalent. Heat is a new form in which a quantity of 

 work may appear. 



These facts no longer permit us to regard heat as a 



