Liquids on the Surface of Water. 41 



21. The spreading of a drop of a liquid upon the surface of 

 another liquid is dependent on the conditions enunciated by Pro- 

 fessor Van der Mensbrugghe in the following proposition : — 



When on the surface of a liquid, A, whose tension = a, we de- 

 posit a drop of a liquid, B, whose tension, b, is notably smaller than 

 a, this drop usually spreads into a thin film with a display of 

 beautiful colours. But if b is superior to, or scarcely inferior to 

 «, the drop, instead of spreading, assumes and maintains a lenti- 

 cular form. Hence it follows that if a liquid, B^ is capable of 

 spreading on another liquid, A, it is not possible that A should 

 spread upon B. 



22. In order to show the superior tension of that portion of 

 the surface not covered by the film, the author resorted to the 

 ingenious contrivance of the silken coil described in my former 

 paper 14 . The action of ether and also of camphor within and 

 without the coil has already been given 15 . One more example 

 will repay perusal. 



A, distilled water ; B, solution of at least one part of Mar- 

 seilles soap in forty parts of water (6 = 2'83); =0*61. 



As soon as a drop of the solution touches the liquid surface 

 within the coil, this immediately assumes the circular form ; 

 but it does not rebound as in the case of ether, because the 

 soapy solution, not being very volatile, does not increase the 

 contractile force by the cold incident on evaporation ; and the 

 oily matter of the soap greatly diminishes the tension of that 

 part of the water on which the drop spreads. It is curious to 

 remark that the floating disk of liquid bounded by the coil moves 

 over the surface of the water, and even rotates, without changing 

 its circular form. These effects are thus accounted for: — If the 

 boundary limited by the coil stopped all communication between 

 the lamina of soapy water and the surface of the pure water, it 

 would be tended equally in all directions by the external liquid 

 layer, and so would remain in equilibrium ; but along this boun- 

 dary there exists generally one or several points where it is pos- 

 sible for the soap-disk to communicate with the pure water 

 outside the coil; and it is here that a small portion of the soap- 

 solution escapes from the coil in a certain direction, and neutra- 

 lizes in this direction the external traction which would otherwise 

 act on the coil at these points. Consequently the forces to which 

 the system is subject are not everywhere and in all directions 

 equal among themselves, so that equilibrium cannot be main- 

 tained. It is easy to conceive that the sum of these forces may 

 be replaced by a single resultant, and then the coil would un- 

 dergo a movement of translation ; or by a couple, when we should 

 14 Phil. Mag. for December 1869, §31. » Ibid. §§ 32 & 35. 



