A N N A L S 



OF 



PHILOSOPHY- 



JUNE, 1821 



Article I. 



A Mathematical Inquiry into the Causes, Latvs, and principal 

 Phcenomena of Heat, Gases, Gravitation, ^x. By John He- 

 rapath, Esq. (In a Letter to D. Gilbert, Esq. MP. VPRS. &c.) 



{Concluded from p. 351 .) 



General Scholium, 



Sudden condensation in all gaseous or aeriform bodies pro- 

 duces heat, and sudden rarefaction cold ; but if the condensation 

 or rarefaction be made slowly, no perceptible change in the 

 temperature takes place. These are natural consequences of 

 our theory of the constitution of gases. For if it be a condensa- 

 tion, by the motion inward, for instance, of one of the sides of 

 the containing body, it is evident that the particles which strike 

 against this side and are reflected back among the rest, will no 

 longer be reflected by the same force only, wdth which they 

 were previous to the commencement of the condensation, but by 

 a force, which will augment their velocity individually by a 

 quantity equal to the velocity of the moving side. And this 

 excess of velocity being distributed to the rest of the particles, 

 or communicated to them by the continuance of the stroke, will 

 generate an excess of temperature throughout the medium pro- 

 portional chiefly to the velocity of condensation. On the con- 

 trary, in the case of rarefaction, those particles, instead of 

 returning among the rest with the celerity with which they did 

 before the side began to move, will now return with a diminu- 

 tion of their velocity, equal to the velocity of the moving side. 

 This diminution must, therefore, cause a decrease of tempera- 



New Series, vol. k 2 c 



