Temperature, and Volume in Saturated Vapours. 303 



f dp 

 Values of - -p for different vapours. 















« 





Pressure in 

 millims. 



J? 



Q £ 











o 



o ©" 



"a? •, 



100 



3426 



3394 



7053 





5218 



5049 





500 



3324 



3329 



7231 



8437 



4995 



4909 





1000 



3313 



3321 



7247 



8275 



4932 



4956 





5000 



3220 



3233 







7936 



4745 



4610 





10,000 











4670 



4528 



1889 



20,000 











4570 



4455 



1991 



50,000 















2018 



This suggested that probably an empirical relation of this 

 kind could be found expressing the relation of pressure and 

 temperature very simply and with considerable accuracy. 



£2-25 fa 



Let, for instance, for steam the values of - ■£ be formed. 

 We get : — 



P 



dt 



t 



P- 



t 2 ' 25 dp 





p dt' 



273 



4-60 



21660 



298 



23-55 



21970 



323 



91-98 



22020 



348 



288-5 



21940 



373 



760-0 



21870 



398 



1743-9 



21830 



423 



3581-2 



21790 



448 



6717-4 



21770 



473 



116890 



21950 



Where the greatest and least values differ from the mean by 

 only 0*7 per cent. But the expression, the values of which 

 have been found, is integrable. Putting n+1 for the index 

 of t, we get for a general approximate relation between the 

 pressure and temperature of vapours, 



i £ 



log e p = a--. 



..... (1) 



From this are easily derived the following equations, in 

 which for convenience the logarithms are common logarithms. 



logp = a-~ (2) 



