6 Mr. Ivory on the Laws of the Condensation and Dilatation 



resume its primitive state when the heat of combination va- 

 nishes. 



But although the density of a mass of air depends solely 

 upon the heat combined with it in a latent form, the tempera- 

 ture will be affected by the variations of density, as well as by 

 the heat flowing from foreign sources. It is lessened, at least 

 for a moment of time, by all the degrees absorbed when air 

 is rarefied, and increased, for a moment, by all the degrees 

 disengaged when air is condensed. Let T denote the actual 

 temperature, and fl the heat received from extraneous sources ; 

 then, T = T + z + fl. 



This formula includes all the possible sources of tempera- 

 ture, but the difference between the quantities represented by 

 i and d must be carefully marked. In the circumstances that 

 generally occur in nature, & is a constant and perpetual cur- 

 rent of heat keeping up the temperature to a determinate level. 

 On the other hand, i is sensible only while the air is actually 

 underi-oing changes of volume, and is soon lost in the general 

 influx of heat after the density becomes constant. When this 

 happens, the temperature is independent of the heat of com- 

 bination. If we suppose that air changes its volume and re- 

 tains the whole of its absolute heat while it is exposed to no 

 extraneous temperature, then 6 = 0. In the atmosphere i 

 and 9 ai'e quantities of a like description, always comparable 

 and dependent on one another. These two cases are, if I 

 mistake not, the only ones that have hitherto been observed 

 in which the elasticity, the density, and the temperature of a 

 mass of air are all ascertained when one is found, and conse- 

 quently in which all of them are functions of the heat of com- 

 bination. 



The known laws of elastic fluids will now furnish these 

 equations, viz. 



T = T + i + 6. 



2. We must next endeavour to determine the form of the 

 function f. This may be accomplished by means of certain 

 experiments made by MM. Clement and Desormes and 

 MM. Gay-Lussac and Welter. The two first-mentioned 

 philosophers filled a close vessel with air, varying in no re- 

 spect from the external air, the height of the barometer being 



equal 



