11Q 



1874.] -L-Li/ [Marsn. 



Owing to the uncertainty as to the exact ratio of the two specific heats 

 of airj and as to the exact value of the mechanical equivalent of heat, 

 an accurate determination of the value of x cannot yet be reached ; but 

 since the above value is based upon the complete series of experiments 

 made by Joule in 1849, it must be entitled to consideration as a fiist ap- 

 proximation, and may be used to illustrate the action of latent heat in 

 the production of luminous phenomena. 



But we are here met by the assertion of several standard writers, that 

 the existence of latent heat of expansion was positively disproved by a 

 certain experiment performed by Joule, who announced in the Philos- 

 ophical Magazine for May, 1845, that "no change of temperature occurs 

 when air is allowed to expand in such a manner as not to develop me- 

 chanical power." 



Although the interpretation thus put upon Joule's words seems to be 

 perfectly natural and legitimate, an examination of the memoir in 

 which he describes his experiments, and announces this conclusion, seems 

 to show that he did not intend them to be so interpreted. 



Prof. Balfour Stewart, in his " Treatise on Heat " (1871, p. 317) says : 

 "Many familiar experiments show that when a gas is suddenly com- 

 pressed, there is a production of heat, and that when suddenly exi:»anded 

 there is an absorption of heat. 



Seguin and Mayer had already suggested the use of gases and vapors 

 for the purpose of determining the mechanical equivalent of heat ; and 

 air, the substance chosen by Mayer, was no doubt very good for such a 

 purpose ; nevertheless, the suggestions of these philosophers do not seem 

 to have been accompanied with a clear appreciation of all the data 

 necessary to a complete proof. 



Joule, however, in his experiments, supplied what was wanting in 

 order to derive a good determination of the mechanical equivalent of heat 

 from the known gaseous laws. By compressing air forcibly into a re- 

 ceiver surrounded by water he found that the water was considerably 

 heated. It is not, however, correct to infer without further experiment 

 that the amount of heat produced in this case is the exact equivalent of 

 the energy expended in compressing the air. A familiar instance will 

 make this clear. By a blow of a hammer upon a small quantity of fulmi- 

 nating mercury, it is exploded, and produces a consideiable amount of 

 heated gas, but we are not at liberty to suppose that all the heat thus 

 developed is merely the mechanical equivalent of the energy of the blow, 

 as will be evident by supposing such an extreme case as a ton of fulmina- 

 ting powder. 



Evidently the substance is in a different molecular condition at the end 

 of the experiment and at the beginning, audit may be supposed with 

 much truth that the heat produced is nearly all due to the conversion 

 into a kinetic form of a certain potential energy present in the compound . 

 Now in the experiment above described, in which air is compressed, the 

 air is evidently in a different molecular condition after compression, for 



