434 On the Determination of the [Dec. 



of heat absorbed, or given out, during this change, determine 

 the whole quantity of heat which the body contained. Thus, 

 supposing the specific heat of ice 09, and that of water 1.0, 

 and admitting that water in freezing gives out 135° of heat, it 

 has been concluded that the real zero is 1350° below the freezing 

 point. 



Different motives, which it would be too tedious to state here, 

 but some of which we shall notice below, have induced philoso- 

 phers to entertain doubts respecting the justice of the principles 

 upon which this determination depends. We are disposed to 

 consider these doubts as well founued. At the same time, we 

 think it worth while to exhibit a calculation of the same kind 

 derived from our experiments. 



It is well known that the theory of sound, founded on the 

 knowledge of the elasticity of the air, gives for ihe rate of its 

 propagation a velocity less considerable than we obtain from 

 experiment. M. de Laplace conceives that this difference 

 between theory and observation is owing to having neglected the 

 increase of elasticity occasioned by the heat evolved in conse- 

 quence of the sudden compression of air during the transmission 

 of sound : and M. Poison has proved that, to make the results 

 of theory and observation agree, we must suppose that a volume 

 of air, when it is suddenly compressed by a force capable of 

 diminishing its bulk -prg-th part, gives out a quantity of heat 

 sufficient to raiseits temperature 1° centrigade (l"b° Fahr.). This 

 supposition is so probable, that we may consider that measure of 

 the quantity of heat disengaged by the compression of air as 

 more exact than what we could obtain by a direct experiment. 

 We may therefore employ this datum to determine the quantity 

 of heat which air would have abandoned in passing from the 

 pressure of 29" 154 inches of mercury to that of 41 "654 inches, 

 to both which pressures it was subjected in our experiments. We 

 find that it would have been sufficient to have raised the tempe- 

 rature of the same quantity of air 54*9°.* Now let us denote 

 by b x the absolute quantity of heat contained in the mass of air 

 compressed, b being its specific heat, and x the excess of its 

 temperature above the real zero. Let us denote by a i the 

 quantity of heat contained in the air not compressed, a being 

 its specific heat. The excess of one of these quantities above 

 the other will be (a — I) x; and since, according to the theory 

 of Dr. Irvine, this excess is equal to the quantity of heat dis- 

 engaged during the change of state of the body, we may repre- 

 sent it in the present case by G54f>°, since b is the specific heat 

 of the compressed air. Therefore (a — b) x = 54 # 96. This 



* Supposing the dbengagflment of heat proportional to the absolute dimi- 

 mi ion of volume, 



