some Thermodynamical Relations. 523 



above 7 atmospheres are much higher than those calculated 

 from Regnault's formula, the difference at 140° amounting 

 to no less than 11*3 atmospheres. 



Where this decrease is observable in the third set of differ- 

 ences, it seems therefore justifiable to doubt the accuracy of 

 the results at the higher pressures. 



It may be stated in conclusion: — (1) That the values of 



-p t are approximately the same for all stable substances at 



the same pressure, but that the differences are real, and are 

 not due to errors of experiment or calculation ; and (2) that 



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



the same for all stable bodies, at any rate for pressures between 

 150 and 2000 millim., while for alcohol and water it is the 

 same for all pressures between 150 and 20,000. 



In the tables which follow, the values of ~ t (absolute 



dp . dt * 



temperature), and J^ . t are given for a number of stable 



substances at definite pressures, also the reduced values of 



-~. t, that for (1) water and (2) carbon bisulphide being made 



= 1*000 at each pressure, the values for other substances at 

 the same pressure being reduced in the same ratio. 



The second series of tables contains similar data for several 

 substances which dissociate more or less completely on their 

 passage into the gaseous state. It will be seen that for such 



bodies the values of ~- . t at any pressure are considerably 



higher than for stable substances at the same pressure. 



The initial letter of the name of the observer of the vapour- 

 pressure of each substance is given in the table containing the 



values of -p at the foot of the vertical columns. R. stands for 

 at 



Regnault, R. and Y. for Ramsay and Young, O for Olszewski, 



N. for Naumann, I. for Isambert, M. & E. for Moitessier and 



Engel. 



