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VII. Some Thermodynamical Relations. — Part II. 

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



IN a note at the conclusion of the first of this series of 

 papers, it was stated that the absolute temperatures of 

 certain nearly related bodies corresponding to equal vapour- 

 pressures are constant at all pressures. Further investigation, 

 however, has shown that although it is only in the case of 

 nearly allied substances, such as chlorobenzene and bromo- 

 benzeue, that the ratio of the absolute temperatures corre- 

 sponding to the same vapour-pressure is a constant, whatever 

 (within the limits afforded by experimental data) that pressure 

 may be, yet a relation does exist between the ratios of the 

 absolute temperatures of all bodies, whether solid or liquid 

 and whether stable or dissociable, which may be expressed in 

 the case of any two bodies by the equation 



W=R+c{t f -t), 



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

 bodies corresponding to any vapour-pressure, the same for 

 both ; R' is the ratio at any other pressure, again the same 

 for both ; c is a constant which may be or a small + or — 

 number ; and t f and t are the temperatures of one of the bodies 

 corresponding to the two vapour-pressures 



When c = 0, R' = R, or the ratio of the absolute temperatures 

 is a constant at all pressures ; and where c is greater or less 

 than 0, its value may readily be determined either by calcula- 

 tion, or graphically by representing the (absolute) temperatures 

 of one of the two bodies as ordinates, and the ratio of the 

 absolute temperatures at pressures corresponding to the abso- 

 lute temperatures of that body as abscissae. It is found that 

 in all cases the points representing the relation of the ratio of 

 the absolute temperatures of the two bodies to the absolute 

 temperatures of one of them fall in a straight line. This is 

 illustrated in some of the examples which are brought for- 

 ward to prove the truth of the law. It follows from this 

 that, if we know accurately the vapour-pressures of one 

 substance, we only require two, or, better, three, accurate 

 determinations of the vapour-pressure of any other substance, 

 at temperatures moderately far apart, in order to be able to 

 calculate the vapour-pressure of that substance at any required 

 temperature, or, rather, to calculate the temperature corre- 

 sponding to any vapour-pressure within the limits of pressure 

 comprised in the determinations of the standard substance. 



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

 Phil. Mag. S. 5. Vol. 21. No. 128. Jan. 1886. D 



