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



lowering of the level of the liquid below the rim of the 

 containing vessel. 



But it is of primary importance to note exactly the con- 

 ditions under which these formulae are applicable. As we 

 have said above, the common assumption is that evaporation 

 is proportional to area, and even where the linear law is 

 assumed, it is often supposed that the law applies only to 

 surfaces of small area. If, however, the conditions specified 

 by Stefan be fulfilled, the law of evaporation is the same, 

 whatever be the dimensions of the evaporating surface. 

 These conditions are given by Stefan in the following 

 words :- — 



" In einer unendlichen Ebene, welche keinen Dampf 

 aussendet, auch keinen absorbirt oder durchlasst, befindet 

 sich eine Vertiefung, welche mit einer Fliissigkeit derart 

 gefiillt ist, dass das Niveau der Flussigkeit mit dieser Ebene 

 zusammenfallt. Die Flussigkeit verdampft in die oberhalb 

 der Ebene befindliche unbegreuzte Luft." 



It is clear that these conditions are not usually fulfilled 

 in ordinary every-day cases of evaporation, and that a better 

 approximation to the conditions is afforded by a number of 

 small apertures pierced in a plate than by a ten centimetre 

 crystallizing dish full of water ; and it is this fact that has 

 probably given support to the idea that Stefan's formulae 

 are approximations applicable to surfaces of the order of 

 magnitude of a few square millimetres. Moreover, Stefan's 

 equations are based on the fact that the lines of flow and of 

 equal vapour pressure are similar to the lines of force and 

 the equipotential lines of the analogous electrostatic problem ; 

 this requires a homogeneous and steady atmosphere over 

 distances from the aperture comparable with its dimensions, 

 and, as Brown and Escombe have pointed out, such con- 

 ditions are more likely to hold good in the case of small 

 surfaces than of large ones. 



Various experiments have been made to test the validity 

 of these results. Srenewsky * pointed out that the evapo- 

 ration from drops varied as their linear dimensions, but the 

 earliest experiments designed actually to test Stefan's 

 formulas were made by Winkelmann f, who observed the 

 rate of evaporation from small vertical capillary tubes con- 

 taining benzol, which were closed at their lower ends and 

 immersed in a circular basin filled with the same liquid. 

 It was found that the rate of evaporation from such tubes 



* Beibl. d. Phys. vii. p. 888 (1883). 

 t Wied. Ann. xxxv. p. 401 (1888). 



