Variation in the Pressure of Saturated Vapours. 63 



We find from the equation 



c— Cj __ 

 AD""* 

 that 



c-c 1 =#xAD = 2-77986x ^=0*07513. 



According to Regnault c 60 = 0*56451, c x = 0*47966; 

 c—c 1 = 0'08485 *. The difference between these two values 

 is considerable, but a closer agreement is not to be expected, 

 for with the existing calorimetric methods the specific heats 

 of liquids, and especially of their vapours, may contain great 

 errors, so that it is doubtful whether they would impose 

 confidence if they were near to each other — to the difference 

 of their c — c-i : if, for example, the second figures (i.e. the 

 hundredth parts) of c and c± contained errors, then in the 

 difference c — Ci the error would be in the first figure, and 

 consequently the figure found by subtracting c L from c would 

 have no scientific value. Errors in the third figures have, 

 although a less, still a great influence on the accuracy of 

 c—Ci. Therefore it is better to determine c x by means of x 

 than by the usual calorimetric method. Taking c = 0*56451 

 and c— Ci=0*07513, we find that c, = 0*48938; this value 

 deserves greater confidence than "47 96 6, found by Reg- 

 nault from experiment. 



After making the necessary substitutions in equation (9) 

 we obtain 



y- 3318-74, 

 and further 



r =3318*74x j? =89*688. 

 u 74 



I calculated from RegnauhVs formula that 



^0=86*196. 



Although there is not an entire coincidence in the results 

 of calculation and observation, still one cannot but acknow- 

 ledge that the figures are very similar. 



2nd Method. 



We take the vapour-pressures corresponding to 60° and 



six adjacent temperatures, and calculate the differential 



dx) 

 coefficient-^- by equation (11). 



* All the data found by Regnault which are here and afterwards 

 inserted are chiefly taken from Laudolt's tables ; the same may be said 

 concerning the data of other observers. 



