Variation in the Pressure of Saturated Vapours. 57 



hire varies constantly by A, then r varies by one and the 

 same quantity (c — <?i)A by equation (2). This temperature 

 will evidently be that same T which was found by the first 

 method. It should, however, be remarked that (c — c^A is 

 exceedingly small compared to r ; therefore, in order to ob- 

 tain results approaching those of experiment, it is necessary 

 to have very exact tables, accurate to tenths, and with small 

 pressures to even hundredths of a millimetre. But it is 

 more than difficult to calculate to such a degree of accuracy, 

 and therefore it is best to employ the first method for deter- 

 mining T and to employ the second method as a control. 



We will apply the second method to aqueous vapour. 

 Broch has recalculated Regnault's observations on aqueous 

 vapour to four points of decimals. I take from them 9 

 pressures for 9 temperatures : for the above determined 70° 



and four above and below it, and I calculate -^- by means 



of the second and third of equations (11). The results are 

 given in the following table ; the temperature t in the first 

 column, the pressure p in the second, and the differential co- 

 efficient -f- in the third. 

 at 



t. 



p. 



dp 



di' 



t. 



p. 



dp 



dT 



o 

 50 



91-9780 

 117-5162 



148-8848 

 187*1028 

 233-3079 



6-923 



8-402 



10122 



o 



75 ..: 



288-7640 

 354-8730 

 433-1938 

 525-4676 



12-107 

 14-388 



55 



60 



65 



70 



80 



85 



90 





I consider it not superfluous to explain how I found 



dpi 



dp 

 di' 



I calculate this quantity for 60°, regarding it as ^~ (11) 



according to the data for temperatures 50°, 55°, 60°, and 65°. 

 The figure 6*9247 is obtained. I calculate the same quantity 



after formula -^- by means of the pressures corresponding to 



