155 EXPERIMENTS ON HEAT AND COLO* 



than that in the fo. When the equilibrium of heat in the air is diflurbed by 



receiver: t j ie p era tions of" condenfation and rarefaction, it is reftored in 



whence in the r _ 



former cafe the the manometer inftautly by reaton of the contiguity of the glats 

 grdual heatin g to the air ; but in the large receiver it requires a fenfible time 

 receiver renders of 1 feconds or more to reftore the equilibrium throughout 

 Us prefTure from the whole internal capacity. It is this reftoration that in- 

 onSemereury e : creafes or diminifhes the elafticity of the air confined in the 

 and the contrary receiver, and thereby caufes the retrogradation of the mercu- 

 in the other r j a j co i umn> Now I have found by former experiments, that 

 a change of 50° in temperature effects a change of T % nearly, 

 in the capacity or bulk of air. It follows therefore that in the 

 cafe of reftoring the equilibrium in condenfed air, about 50 p 

 of cold is propuced ; and in letting in air to an exhaufted re- 

 ceiver fomething more than 50 Q of heat is produced. The 

 (mall difference feems to arife from this, that the condenfation 

 of vapour in the former cafe diiiiinijhes the effect, and in the 

 Jatter, if any there be, increafes the effect, that would arife 

 from operating upon purely dry air. 

 The theory of The experiments and obfervations hitherto related go prin- 

 thefe effects c jp a i]y ^ a fcertain facts without any reference to the theory 

 ference of capa- of them : This however may be given in a few words, and is 

 city of air for the f ame t h at ; s a f cr ibed to Mr. Lambert by Meats. Sauffure 

 the greater tbt and Pictet and by them adopted. He conceives that a va- 

 tdtnjty. cuum has its proper capacity for heat, the fame as air, or any 



other fubftance ; and that the capacity of a vacuum for heat 

 is greater than that of an equal volume of atmofpherieal air ; 

 alfo that the denftr air is, the lefs is its capacity for heat : upon 

 thefe principles the phenomena are eafily referable to that 

 clafs of chemical facts where heat and cold are generated by 

 the mixture of two different bodies. If this theory be right, 

 Whence new and I think there is little doubt of it, we may hence be led 

 experiments may j nt0 a tra ; n of -experiments, by which the abfolute capacity of 

 the' capacities of a vacuum for heat may be determined ; and likewife the ca- 

 elaftic fluids. pacities of the different gafes for heat, by a method wholly 

 new : but this mud be left to future invefiigation. 



VI. An 



