604 



Sir William Thomson 



[Jan. 29, 



microscope, we can make the necessary measurements on real drops 

 of water or other liquid, for the purpose of determining the values 

 of the capillary constants. For stability the drop hanging from the 

 open tube should be just less than a hemisphere, but for convenience 

 it is shown, as in the enlarged drawing of the nozzle (Fig. 30), 



Fig. 30. 



exactly hemispherical. By means of the siphon the difference of 

 levels, h, between the free level surface of the water in the vessel to 

 which the nozzle is attached, and the lowest point in the drop 

 hanging from the nozzle, may be varied, and corresponding measure- 

 ments taken of Ji and of r, the radius of curvature of the droj) at its 

 lowest point. This measurement of the curvature of the drop is 

 easily made with somewhat close accuracy, by known microscopic 

 methods. The surface-tension T of the liquid is calculated from the 

 radius, r, and the observed difference of levels, Ji, as follows : — 



2T 



= h 



for example, if the liquid taken be water, with a free-surface tension 

 of 75 milligrammes per centimetre, and r = "05 cm., h is equal to 

 3 centimetres. 



Many experiments may be devised to illustrate the effects of 

 surface-tension when two liquids, of which the surface-tensions are 

 widely different, are brought into contact with each other. Thus wo 

 may place on the surface of a thin layer of water, wetting uniformly 

 the surface of a glass plate or tray, a drop of alcohol or ether, and 

 so 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 surface-layer of alcohol largely diluted with water we may arrange 

 to withdraw part of the alcohol at one particular place by promoting 

 its rapid evaporation, and thereby increase the surface-tension of the 



