Temperature, and Volume in Saturated Vapours. 301 



we get from the values of -J- in Messrs. Ramsay and Young's 



T dp 



tables the following values of /-: — 



& p at 



Substance ... \ 



Zero | 

 (Above -273°C) / 



Bisulph. of 

 Carbon. 



144°. 



Ethyl 

 Alcohol. 



208°. 



Water. 

 210°. 



Mercury. 

 322°. 



Pressure in mm. 



100 



500 



1000 



5000 



10,000 



20,000 



5-913 

 5-749 

 5-666 

 5-201 



5-323 

 5-616 



5-802 

 5-540 

 5-429 

 5-291 



5-183 

 5-794 

 5-808 

 5-645 

 5-503 

 5-302 



5-243 

 5-598 

 5-707 



At first sight it looks as if this were similar to the relation 



t -j- = constant at any one pressure, given in Messrs. Ramsay 

 at 



and Young's paper. But, obviously, it is a quite different 

 relation from the difference in the reckoning of temperature. 

 It is, besides, a purely empirical relation, and the constancy of 

 the values obtained may serve as a useful warning against 

 accepting such correspondence of formula and data as indi- 

 cating that the formula expresses a real law. 



The principal relation discussed by Messrs. Ramsay and 

 Young is that 



t dp 

 dt 



is constant for different vapours at the same pressure. In 

 their table (Proc. Phys. Soc. vol. vii. p. 301*) values of this 

 quantity, very carefully and laboriously calculated, are given 

 for various vapours at pressures ranging from 10 to 20,000 

 mm. of mercury. It will easily be seen that the numbers in 

 each horizontal line in their table approximate to constancy, 

 varying in extreme cases by possibly 30 per cent. 



If now these numbers are divided by the pressures so as to 

 form values of 



t dp 

 p dt 



the constancy of the horizontal lines of figures will remain 

 unaffected, while the vertical columns will approach to con- 

 stancy almost as nearly as the horizontal columns. The 

 following table gives these values, with some of those for 

 mercury corrected and with those for vapour of carbonic 

 anhydride added: — 



* Phil. Mag. vol. xx. p. 526 (December 1885). 



