520 Drs. Ramsay and Young on 



In the calculation of the vapour-pressures of a number of 

 substances for each degree between certain limits of pressure, 

 it became evident that at any given pressure the rate of in- 

 crease was generally, though not always, greater for the 

 volatile substances than for the less volatile. 



It was found that the product of the absolute temperature 



into the rate of increase of pressure ( -^ . t\ at any given 



pressure was approximately the same for the bodies examined, 

 but the differences were evidently too great to be ascribed to 

 errors of experiment or of calculation. That this product 

 should be approximately the same for different substances 

 might perhaps be anticipated from the following considera- 

 tions : — If there are two bodies, the absolute temperatures of 

 which must be raised to 200° and 400° respectively, in order 

 to produce a certain effect, the same for both, it might be 

 expected that a further rise of temperature of 1° would pro- 

 duce a greater effect on the substance whose temperature was 

 200° than on that at 400°; for the rise of temperature in the 

 first case is greater in proportion to the temperature to which 

 the body has already attained than it is in the second, the 

 rise in the one case being from 200° to 201°, and in the other 

 from 400° to 401°. The rise of temperature would perhaps 

 rather be proportional if the temperature of the hotter body 

 were raised from 400° to 402°, or if the rise of temperature 

 in each case were made proportional to the absolute tempera- 

 ture. In order, then, to make the conditions as similar as 



possible in the calculation of the value of ~j- for different sub- 



dt 



stances, the magnitude of the unit degree of temperature 

 should be made to vary in the same ratio as the absolute 

 temperature of those substances, corresponding to the par- 

 ticular pressure at which they are compared ; or, in other 



words, keeping the unit degree constant, the value £ should 



be multiplied by the absolute temperature t 



dx) 

 The values of ~ . t were determined for a number of liquids 



at several different pressures ; and it was found that the pro- 

 ducts obtained for different stable substances at the same 

 pressure were always approximately the same, whatever that 

 pressure might be. Thus at a pressure of 400 millims. the 

 following values were obtained : — 



