EESEARCHES ON EVAPOEATION AND DISSOCIATION. 315 



1885, p. 515 ; 1880, pp. 33, 135 (vol. i.), p. 32 (vol. ii.)~-In 

 the tliermodynamical equation, 



L _ dp T 

 Sj— Sg dT ' J 



(7"j=koat of vaporization : ,';^=vol. of unit mass of saturated 

 vapour; 83= vol. of unit mass of liquid ; i|^=rate of in- 

 crease of ])ressure per unit rise of temperature ; .7"=mecliani- 

 cal equivalent of heat), the following relations hold : (1) 

 The amount of heat required to produce unit increase of 

 volume in the passage from the liquid to the gaseous state,' 

 at the boiling point under ordinary pressure, is approxi- 



mately constant for all bodies, 



L 



(2) If the 



amounts of heat required to produce unit increase of 

 volume in the passage from the liquid to the gaseous state 

 bo compared at different pressures for any two bodies, then 

 the ratio of tJio amoiint at the boiling-point under pressure 

 ^5j, to the amount at another pressure p^, is approximately 

 constant for all liquids. It follows, that the external and 

 total work bear an approximately constant ratio to each 

 other at any one pressure, whatever be the liquid. (3) The 



values of 



dp 

 dT 



T are approximately the same for all stable 



bodies at the same pressure; but the differencos are real, and 

 are not duo to errors of experiment or of calculation. (4) 



The rate of increase of this value, '-£^^ T, with rise of pres- 

 sure, is the same for all stable bodies, at any rate for 

 pressures between 150 mms. and 2,000 mms. The lirst and 

 third, and the second and fourth of these laws are identical, 

 but the proof rests on an entirely different experimental 

 basis, and is mucli more complete for the pressures than for 



