78 Mr. E. P. Perman. [Feb. 4, 



Pressure of aqueous vapour = Volume of aqueous vapour ^ 



Total pressure Total volume 



The " normal " value of the density of aqueous vapour is taken to be 

 that calculated from the density of oxygen* and the molecular weight of 

 water, and the corresponding specific volume, which is more convenient 

 for purposes of calculation, is 1-242. The vapour pressure is calculated 

 then from the expression 



1-242x760 W.P.T 

 T(l-242 x 760W) + 273.V.J) ' 



Results. 



The lowest temperature at which the method was applied was 20 C. 

 The experiments are naturally very tedious at low temperatures owing 

 to the large volume of air which must be passed through the apparatus. 

 The whole of the experimental results are here tabulated, the actual 

 data being given in an Appendix. 



It will be noted that the temperatures are at exactly every 10 

 (with two exceptions); the corrections were calculated beforehand, 

 and the thermostat carefully adjusted. One exception is 80'10, which 

 was the temperature in the only series of experiments made with a 

 portion of the thermometer stem outside the bath ; a correction was 

 made for this exposed portion in the usual way. On calculating out 

 the results, the vapour pressure appeared to be abnormally high, and 

 consequently a large number of experiments were made to ascertain 

 this point with certainty. However, on proceeding to 90, normal 

 vapour pressures were again obtained, and suspicions were aroused as 

 to the exactness of the temperature reading at 80. The thermostat was 

 brought again to 80, and the thermometer placed first with its stem 

 surrounded by the wide tube and hot liquid, and then placed in the 

 original position, when it was found that the correction had been over- 

 estimated by 0-1. The reason for this large difference between the 

 calculated and the real correction is probably that in this class of 

 thermometer the column of mercury projecting from the thermostat is 

 surrounded by a bath of hot air enclosed by the outer tube of the 

 thermometer. The other exception, at 40'66, was one of the earliest 

 experiments carried out. 



The vapour pressures given in the last column are taken from 

 Ostwald and Luther's ' Physiko-chemische Messungen ' (1902), 

 p. 156. The numbers were obtained by plotting the differences 

 between the results of Wiebe (76 to 100) and Thiesen and Scheel 

 (- 10 to +25), and the Kegnault-Broch numbers, interpolating the 

 curve from 25 to 76, reading off the corrections from the smoothed 

 curve, and applying them to the Kegnault-Broch numbers throughout. 

 * The weight of a litre of oxygen has been taken as T4295 g. 





