On the Mechanical Effect of Heat and of Chemical Forces. 7 



stitutes thermometric heat, the hypothesis which I have called 

 that of molecular vortices, that is to say, if we suppose thermal 

 elasticity to arise from the centrifugal force of molecular revo- 

 lutions, I have shown that the functions a and Z> are propor- 

 tional to the absolute temperatures diminished by a constant 

 which is yet unknown, but which must be the same for all sub- 

 stances; being, in fact, the absolute temperature which cor- 

 responds to total privation of heat ; in other words, the distance 

 between the absolute zero of gaseous elasticity and the absolute 

 zero of heat. This is expressed by the formula 



a—h ___ rg—Tb 



f 



Ta^K 



where Ta and r^ are the absolute temperatures, and k the constant 

 in question. If we assume that this constant is either null or 

 inappreciably small compared with the temperatures at which 

 machines usually work, we obtain the formula 



Ta—n 



which you have employed in your investigations. 



With respect to the value of the constant /c, the only existing data 

 which throw any light upon it are the experiments just published 

 by Thomson and yourself on the heat produced by the friction 

 of air j and even they, so far as they have yet gone, are sufficient 

 to prove only that it is exceedingly small. When carried out on 

 a larger scale (and especially with carbonic acid gas, for reasons 

 which 1 shall point out), I have no doubt they will furnish the 

 means of determining it exactly. 



I am now deducing the theoretical results of these experiments 

 according to the hypothesis of molecular vortices ; and though 

 my calculations are not yet complete, I think it desirable to let 

 you know the results so far as I have gone. 



The mechanical value of the heat which disappears in the first 

 instance in expanding a substance from the volume Vj to the 

 volume V2 at a constant temperature, is represented by 



i'-')/,-f"^ 



The power developed by the expansion, which in your experi- 

 ments is reconverted into heat by friction, is 



Vi 



