﻿of the common Surface of two Liquids. 4(57 



fied layer of oil which covers the water or mercury has relatively 

 a surface tension a 2 or a 2i> Dotn °f which are ^ ess than the sur- 

 face-tension u x of the free surface of the water or mercury 

 covered with the much modified layer of oil. Liquid 2 will re- 

 main on the surface of liquid 1 as lens-shaped drops with various 

 angles, or spread itself out, according to the magnitude of the 

 constants a,, a 2 , and a 21 , as experiment also shows. 



Fig. 4 gives the cross-section of a drop of oil on a surface of 



Fig. 4. 



-W<xte-r- 



water containing oil. The form of the same changes with the 

 quantity of oil already dissolved by the water and the nature of 

 the film of oil which covers the water. The drop generally be- 

 comes flatter the longer it remains on the water. 



By breathing on the oil-lenses swimming on water, there may 

 be observed, when the surface is sufficiently mobile, a similar 

 contraction of the oil-lenses and change of the angle to those I 

 have described in § 27, when water lenses on mercury are breathed 

 on. The phenomenon is accounted for here, as there, by sub- 

 stances which are contained in the form of vapour in the air ex- 

 pired and are condensed on the surface of the water. 



As with water and mercury, so also with oil and water, thin 

 layers of oil with circular holes occur whose angle is the same 

 as that for a lens of oil on the same surface of water. 



By means of the foregoing, the interesting phenomenon is 

 explained which is observed when a drop of oil spreads itself 

 out on a large surface of water, for instance, on a pond. A 

 portion of the oil spreads itself out as a layer on the surface 

 of the water to the extent of several square feet, and forms 

 the most splendid interference-colours. The rest remains in 

 the form of lens-shaped drops in the middle of the coloured 

 oil-film. The latter gradually becomes thinner, draws itself 

 together in a few places as irregular spots, and is gradually 

 absorbed by the water. If the film of oil around the lens- 

 shaped drop becomes infinitely narrow, then the latter sud- 

 denly spreads upon the free surface of the water ; I might say 

 the oil-lens explodes and forms a new film of oil of uniform 

 colour or thickness, and from 2 to 3 inches in diameter. After 

 this the process described above again repeats itself. This 

 experiment sometimes succeeds with small drops of oil on a 

 plate filled with pure water. 



30. Besides those described in the foregoing paragraphs, a 



