[ 135 ] 



XVIII. Some Thermodynamical Relations. — Part III. 

 By William Ramsay, Ph.D., and Sydney Young, D.Sc* 



IN the first of this series of papers (Phil. Mag. Dec. 1885) 

 two relations between the vapour-pressures of different 

 substances were described: — 1. The product of the rate of 



increase of pressure per unit rise of temperature (-j-) into 



the absolute temperature (t) is approximately the same for all 

 bodies at the same pressure, but the differences are real, and 

 are not due to errors of experiment or of calculation. 2. The 



rate of increase of this value -£ . t with rise of pressure is 



very nearly the same for all bodies. This was shown in the 



tables by making the product -£ . t for one substance equal 



to unity at each pressure, and reducing the values for other 

 substances at the same pressure in the same ratio. It was 

 pointed out that, at pressures between 150 or 200 millim. and 

 1500 or 2000 millim., the variations from constancy are very 

 small, but that they are frequently greater at lower and at 

 higher pressures ; and it was suggested that these variations 

 might possibly be due either to errors of experiment or of 

 calculation. 



In the second paper a relation was proved to exist between 

 the absolute temperature of any two bodies corresponding 

 to equal vapour-pressures, which may be expressed by the 

 equation 



R'=E+c (*'-*)> 



where R/ is the ratio of the absolute temperatures of any two 

 bodies at a given pressure, R the ratio at another given pres- 

 sure, t' and t are the temperatures of one of the two bodies 

 corresponding to those vapour-pressures, and c is a constant. 

 The value of c may be determined graphically by making the 

 temperatures (absolute or Centigrade) of one of the bodies 

 (A) ordinates and the ratios of the absolute temperatures of 

 the two bodies (A and B) at pressures corresponding to the 

 temperatures of (A) abscissae, when the points representing 

 this relation fall in a straight line. 



In the case of nearly related bodies such as chlorobenzene 

 and bromobenzene, c = and R' = R at all pressures, and in 



those cases the ratios of the products •—- . t are also equal at 

 * Communicated by the Physical Society : read December 12, 1885. 



