Pressures, in 



I, 



millim. of 





mercury. 



*l-8 2 * 



300 . . 



. 0-137 



1000 . . 



. 0-405 



2000 . . 



. 0-742 



3000 . . 



. 1-055 



5000 . . 



. 1-633 



some Thermodynamical Relations. 519 



dp t 

 It T 



0-1395 



0-414 



0-770 



1-110 



1-743 



The non-equality of these numbers may be ascribed to 

 various causes : want of accuracy in experiment, most pro- 

 bably in determination of the heats of vaporization, and the 

 assumption that the density of the saturated vapour of water 

 is normal at high pressures, are among the most likely. 

 With such results for water, it was thought unnecessary to 

 compare these terms for other liquids, where the constants 

 are probably less accurately determined. These numbers, 

 however, suffice to prove that the relations pointed out in the 

 first part of this paper, although derived from observations 

 and assumptions not always trustworthy, agree fairly well 

 with those deduced from the vapour-pressures, which have 

 been determined with a high degree of accuracy. 



We now proceed to exhibit the relations between the 

 vapour-pressures of various liquids. The constants of the 

 bodies considered were determined by Regnault, Olszewski, 

 Naumann, Isambert, Moitessier and Engel, and by ourselves. 



If a curve be constructed to represent the relation of tem- 

 perature to pressure for any substance, and if tangents be 

 drawn to touch the curve at various points corresponding to 

 certain temperatures, these tangents will give the rate of 

 increase of pressure per unit rise of temperature ; in other 



words, the value y- for those temperatures. 



If we construct curves for a number of substances, and 



determine the value of -r- for each of them at the same tem- 

 dt 



perature, it is clear that the values obtained will differ widely, 



and will be greater for volatile substances than for those 



which are less volatile. But if we determine the values of 



^ for the same series of bodies, not at the same temperature, 

 at ; 



but at the same pressure, the conditions under which the 



comparison is made will be more similar, and the resulting 



values may be expected to differ much less. 



202 



