48 POPULAR LECTURES AND ADDRESSES. 



cause the surface-tension of the liquid layer to 

 become smaller in the region covered by the 

 alcohol or ether. On the other hand, from a sur- 

 face-layer of alcohol largely diluted with water, we 



may arrange to withdraw part of the alcohol at 

 one particular place by promoting its rapid eva- 

 poration, and thereby increase the surface-tension 

 of the liquid layer in that region by diminishing 

 the percentage of alcohol which it contains. 



