94 Journal of the Mitchell Society. [Nov. 



Young's observations of the vapor pressure of water are in 

 beautiful accord (0° to 230° C) with the Regnault-Biot 

 formula, whose results they quote. But when this formula is 

 forced past the temperature (230° C) for which it was calcu- 



SP 

 lated, an error in the trend ^-^ of the curve is at once 



apparent. 1 This error could neither have been suspected nor 

 provided for, had either set of observations below 230° C been 

 used to calculate the formula. 



In this connection Prof. Young calls my attention to 

 Broch's calculations of the vapor pressure of water, 0° to 100° 

 C. Broch used the formula, 



P = A 10 bt + *' + dP + et * + ft% 



1+ at 



and made exceedingly careful and laborious calculations. 

 But unfortunately in obtaining his constants he used Reg- 

 nault's data from — 32° to 100° and the vapor pressures at the 

 lower temperatures were for the most part those of ice, not 

 water. Hence, remarks Prof, Young: "It is interesting to 

 notice that below 0° his calculated pressures are with trifling 

 exceptions, higher than the vapor pressures of ice observed by 

 Regnault — a striking proof, if such were needed, that the 

 vapor pressures of water are really higher than those of ice 

 at the same temperatures. Moreover, Broch makes the vapor 

 pressure of water at 0° = 4.569, whereas the mean of Reg- 

 nault's actual observations, 12 in number, is 4.608." Again 

 at 100° C, Prof. Young points out that the trend of Broch's 

 curve is certainly wrong, and in proof of this he sends, and 

 kindly permits me to publish, the following calculations upon 

 the vapor pressure of water, made some years ago but never 

 published. 



* See Phil. Trans. 1892A, p. 112. 



