186 THOMSON ON CARNOT'ti 



From this result we draw the following conclu- 

 sion : 



47. Equal volumes of all elastic fluids, taken at 

 the same temperature and pressure, when com- 

 pressed to smaller equal volumes, disengage equal 

 quantities of heat. 



This extremely remarkable theorem of Carnot's 

 was independently laid down as a probable experi- 

 mental law by Dulong, in his " Recherches sur la 

 Chaleur Specifique des Flu ides Elastiques," and it 

 therefore affords a most powerful confirmation of 

 the theory.* 



* Carnot varies the statement of his theorem, and illus- 

 trates it in a passage, pp. 81, 82, of which the following is 

 translation : 



" When a gas varies in volume without any change of tem- 

 perature, the. quantities of heat absorbed or evolved by tJiis gas 

 are in arithmetical progression, if the augmentation or dimi- 

 nutions of volume are in geometrical progression. 



" When we compress a litre of air maintained at the tem- 

 perature 10, and reduce it to half a litre, it disengages a 

 certain quantity of heat. If, again, the volume be reduced 

 from half a litre to a quarter of a litre, from a quarter to 

 an eighth, and so on the quantities of heat successively 

 evolved will be the same. 



"If, in place of compressing the air, we allow it to ex- 

 pand to two litres, four litres, eight litres, etc., it will be 

 necessary to supply equal quantities of heat to maintain the 

 temperature always at the same degree. " 



