Physics. ''On the Evaporation from a Circular Surface of a Liquid'* . 

 By Dr. H. C. Bukger (Communicated by Prof. W. H. Julius). 



(Communicated in the meeting of Deceniber 29, J 917.) 



In a publication recently published Miss N. Thomas and Dr. A. 

 Ferguson^) communicate observations concerning the evaporation 

 from circular water surfaces. These observations are made under 

 different circumstances viz. in a dark, vei'y quiet room, in a lighted 

 room and in the open air. It appeared, that in every case the 

 quantity of water evaporated in unit time, might be represented by: 



Ez=Kr^^, 



in which r is the radius of the water surface and K and n are 

 constants that, except on the external circumstances, also depend 

 upon the distance of the surface of the liquid and the rim of the 

 basin in which this is contained. Now while, as the writers remark, 

 usually in the literature the opinion is found, that the evaporation 

 is proportional to the area of the surface, i. e. that n = 2, it was 

 shown by their experiments that this exponent was always between 

 1 and 2. Now Stefam ^) has treated the evaporation from a circular 

 snrface of a liquid, supposing that the vapour diffuses in the space 

 above the plane in which the level of the - liquid lies, while at 

 the liquid the concentration of the vapour is a constant. 



The result of the computation is, that the speed of evaporation is 

 proportional to the radius of the surface. So it is apparent that in 

 the experiments of Thomas and Ferguson the conditions that Stefan 

 supposes ill treating the problem, are not fulfilled. 



As 1 have already been engaged for some time upon the theoretical 

 and experimental treatment of the diffusion in a flowing liquid *), 

 it was of importance to inquire whether my results agreed with 

 the above mentioned investigations. For this purpose we must extra- 

 polate the values of the exponent n for the case that the surface of 

 the liquid is on a level with the rim of the basin. When this is 



1) Phil. Mag. XXXIV p. 308, 1917. 



2) Wied. Ann. XVII p. 550, 188i2. 



*) My principal purpose in this is lo investigate whether the solution at the 

 surface of the crystal is saturated or if perhaps, when the solving takes place 

 sufficiently rapidl}^ an undersaturalion arises. 



18* 



