Evaporation from a Circular Water Surface. 313 



On the other hand, if we employ the equation 



dpi __ fr d'Pi 



~dt * a^ j 



we are led to a definition of # such as the following : — 

 If layers of equal density are horizontal planes and p is 

 the density of a gas A at a height a above a fixed horizontal 

 plane, then in unit time the mass of A which passes down- 

 wards through unit area of a horizontal plane at a height x 

 is proportional to the density gradient and is equal to k"dpfb&, 

 where k is the coefficient of interdiffusion of the gases 

 A and B *. 



Whichever definition be employed, the dimensions of k 

 are, of course, the same and are those of 



[Surface] 

 [Time] ' 



As we have already mentioned, the equivocal nature of 

 such terms as " amount " has also had considerable influence 

 in introducing confusion. 



Now, considering Stefan's equation for the evaporation 

 from a circular surface, viz., 



V=4falog.|=£, 



examination of the dimensions shows at once that V must 

 stand for the volume evaporated per unit time ; and, although 

 Stefan consistently uses the ambiguous term " Menge," the 

 symbol V shows clearly that the quantity under discussion 

 is volume and not mass. 



If, indeed, we interpret V as meaning evaporation in 

 grams per unit time, an application of the above formula 

 gives results of quite the wrong order of magnitude, as the 

 following calculation shows. 



In one of our experiments a basin of radius 2*08 cm. lost 

 1-727 gins, of water by evaporation in 19 h 41 m under the 

 conditions immediately following : — 



Ht. of barometer (mean), 76*7 cm. 

 Mean relative humidity, 56 per cent 

 Mean temperature, 15° C. or 288° A. 



* See, for example, Poynting and Thomson, ' Properties of Matter/ 

 6th ed. 1913, p. 196. 



