166 Mr. Ivory on the Combination of Heat 



tain our knowledge of it experimental!}', and by experiment 

 it must be limited. MM. Dulong and Petit have compared 

 the expansion of air under a constant pressure with the indi- 

 cations of a mercurial thermometer; and their researches 

 prove that the increase of volume keeps pace with the ascent 

 of the mercury between the points at which mercury freezes 

 and boils, that is, from —40° to about 600^ of Fahrenheit's 

 scale*. We must therefore infer, that for this long range of 

 temperatui'e, equal quantities of absolute heat have caused 

 equal increments of volume both of the air and the mercury. 



In the second place, when heat is applied to air which is 

 kept from changing its volume, the elasticity increases at the 

 same rate with the teuiperature. There is here no absorption 

 or disappeai'ance of any part of the heat; the whole of it is 

 employed in augmenting the elasticity. But although the 

 effect is single, yet, because the condition of the air varies as 

 moi'e heat is applied, we cannot infer that equal increments of 

 elasticity accompanj' equal rises of temperature. This is 

 a point that experience must determine ; and, from the re- 

 searches of MM. Dulong and Petit and other natural philo- 

 sophers, it appears, that the increase of elasticity keeps pace 

 with the rise of temperature within the limits ah'eady men- 

 tioned, that is, from —40° to 600° of Fahrenheit's scalef. 



It is next to be shown that when air under a constant pres- 

 sure expands by heat, the whole heat it acquires is resolvable 

 into two distinct and independent parts; the latent heat which 

 unites with the air as the volume increases without affecting 

 the thermometer, and the heat of temperature which is capa- 

 ble of raising the temperature from the initial to the ultimate 

 quantity, the volume remaining invariable. In order to prove 

 this, it is to be observed, that a given mass of air may be made 

 to change its bulk and temperature in two different ways. 

 First, the pressure remaining invariable, the air may be di- 

 lated to any proposed volume by the direct agency of heat. 

 Secondly, the same mass of air may be allowed to expand to 

 the same volume without the application of heat, either by en- 

 larging the dimensions of the containing vessel, or by lessen- 

 ing the pressure ; and when this is done, the volume being kept 

 from changing, the temperature is next to be raised to the 

 same degree as in the first process. In this second method 

 of operating, heat enters the air as it expands; and as the 

 temperature is the same both before and after the expansion, 

 it follows that the latent heat depends solely upon the increase 

 of volume, and is perfectly distinct from the heat of tempera- 



* Journal de PEcolc Polylcch. torn. ii. p. 200. 

 t Ibid. pp. 199,200. 



ture 



