314 Miss N. Thomas and Dr. A. Ferguson on 



At 0° C. and 76 cm. pressure Winkelmann gives k as 

 0216. Hence we have 



:= Kl) 



T \ 1-8 76 



-=0-236. 

 P 



Also p x = maximum pressure of aqueous vapour at 15° C. = 

 1*28 cm., and therefore from the humidity data p = '12 cm. 

 Substituting these values in Stefan's equation we have 



V = -0143 



as the amount (Menge) evaporated per second. 

 The observed loss in grams per second is 



1 "727 



and it is seen that the two quantities V and E which, if V 

 stands for the evaporation loss in grams per second, should 

 at least be of the same order of magnitude, stand in the ratio 

 of about 600 : 1. 



If, however, it be assumed that V stands for volume lost 

 per second^ the results, though not showing any quantitative 

 agreement, are of the same order of magnitude. 



Nevertheless, in both editions of Preston's ' Heat ' * the 

 symbol V is exchanged for M, and it is definitely asserted 

 that in the equation 



P-/'o 



M.=4ka\oe 



"P-pt 



M stands for the mass of liquid evaporated per unit time — 

 a result which is dimensionally impossible. 



The same error appears to have been made in Waitz's 

 article on " Diffusion " in Winkelmann's Handbuch f, although 

 the use of the term " Menge " makes the results difficult to 

 interpret. Just as in Preston, Stefan's equation is here 

 given with " M" substituted for "V," and as in the general 

 discussion " Menge " is used where mass is undoubtedly 

 meant J, it is probable that the same holds good in its use 

 in Stefan's equation. 



* 1894, p. 291 note, and 1904, p. 357 note. 



t Handbuch d. Phijsik, i. pt. 2, p. 1430. 



X E. g. : " Betrachtet man ein Gasvolumen zwischen zwei urn dx 

 voneinander abstehenden Querscknitten Q, so geht durch den unteren 

 Querschnitt desselbe in der Zeit dt von dem in Richtung der wach- 

 senden .%'.... sich bewegenden Gase die Menge 



dm=-kQ^'.dt." 



QX 



