Variation in the Pressure of Saturated Vapours. 59 



of aqueous vapour may be calculated by means of equation 

 (5) or (7), which in the given instance takes the form 



lo S23fo9 = -[ log -&- m 



: "^M 15-2948 



+ m 



|_T 343J 



5106-2. 



In the following table the temperatures t are placed in the 

 first column, the pressure of aqueous vapour in millimetres, in 

 a state of saturation and calculated by means of equation (7), 

 in the second column, and the pressures according to Reg- 

 nault's observations in the third column*. 



t. 



Pressures. 



Calculated from 



Observed. 



Calculated from 



Calculated from 





equation (7). 



equation (5 2 ). 



equation (5{). 



o 



-30 



0-358 



0-386 



0510 



1067-15 



-20 



0-891 



0-927 



1-170 



817-03 



-10 



2-053 



2-151 



2-519 



817-03 







4-408 



4-569 



5-120 



428-28 



10 



8-949 



9-140 



9-936 



428-28 



20 



17-159 



17-363 



18-392 



187-55 



30 



31-334 



31-510 



32-692 



. 187-55 



40 



54-74 



54-87 



5601 



94-84 



60 



148-86 



148-88 



149-20 



150-15 



80 



354-92 



354-87 



355-70 



353-47 



100 



758-65 



760-00 



772-56 



704-61 



140 



2669-4 



2717-6 



29089 



1851-54 



180 



7187-7 



7546-5 



8667-0 



1851-54 



220 



15864-8 



17390-4 



21629-0 



5921-18 



In this table the temperatures 65°, 70°, and 75° are omitted, 

 because at these temperatures there can be no difference be- 

 tween the results of calculation and observation, as they 

 appertain to points through which both curves pass ; that is, 

 both the curve expressed by equation (7) and that representing 

 the actual variation of the vapour-pressure. The curves, which 

 almost accord between the common points, diverge as they 

 become more remote from them, but in their general properties 

 they diverge very slowly, as is indeed seen from the table. 

 Such temperatures as 40°, 60°, and 80°, for which the pressures 

 are calculated after equation (7) and determined by obser- 

 vation, may be considered as equal. At higher and lower 



* The meaning of the figures in the 4th and 5th columns will be 

 explained presently. 



