230 Royal Society .-—Mr. Tomlinson and Van der Measbrugghe 



any separation of salt, viz. pale seal-oil, olive-oil, rape, castor-oil, 

 croton-oil, niger, sperm, and cotton-seed oil. 



So far this result is in accordance with the theory. 



Experiment 13. A solution of the same strength as in the last 

 experiment was employed, when a drop of seal-oil, sperm, cotton- 

 seed, and niger spread out into films with powerful nuclear action. 

 Linseed- and castor-oil formed lenses with no such action. 



Now it must he remarked that on the day when Exp. 12 was made 

 the weather was dull, damp, and cloudy, and during the time of 

 Exp. 13 the weather was bright and clear. Some years ago it was a 

 matter of frequent observation to one of us, that the formation of co- 

 hesion-figures on the surface of water was much more rapid and de- 

 cisive, with altogether finer and sharper results, in bright weather 

 as compared with dull, damp, wet, or foggy weather. The same 

 remark applies to the motions of camphor on water, and to those 

 curious phenomena known as ''camphor-currents" and ''camphor- 

 pulsations"*. In the production of all these phenomena, as has 

 been shown by one of usf, surface-tension plays a most important 

 part ; and such tension is lowered in dull foggy weather probably 

 by the condensation of the vapour of volatile matters contained in 

 the atmosphere. A drop of a liquid under such conditions may not 

 spread on the surface of water or of mercury, the latter being espe- 

 cially liable to such influences ; whereas on a bright day such sur- 

 faces are particularly active, and experiments succeed which some 

 hours or days before failed to produce the results expected. 



Then, again, as pointed out by one of us in Part II., the viscosity 

 of the surface, or of the drop of liquid placed upon it, may greatly 

 interfere with the operation of the law by which a liquid B spreads 

 upon the surface A. A supersaturated saline solution has a consi- 

 derable viscosity of surface, which it retains for many hours after it 

 has cooled down. In the course of about twenty-four hours the 

 more wateiy particles come up to the surface and the tension im- 

 proves ; so that the same surface which may have sufficient tensile 

 force to cause a drop of oil to spread upon it, might some hours 

 earlier have retained it in the lenticular form:j;. 



There are also certain modifications to which oils &c. are subject 

 in consequence of the presence of ozone and other matters in the air, 

 which may somewhat disturb the results expected to be obtained 

 fronr the action of surface-tension. 



It was stated in Part II. that when an oil &c. assumes the len- 

 ticular form, the solution may be agitated so as to break up the leus 



* Phil. Mag. for Dec. 1869. 



t Sur la Tension superficielle des Liquides, par G. Van der Mensbrugghe. 



t Some of the distinguished physicists Tvho are now engaged in studying the 

 phenomena of surface-tension refer to the embarrassing effects of surfece-vis- 

 cosity. Thus Herr Liidtge remarks that a solution of soap (^=2-8 to 3) does 

 not spread upon a solution of Panama -wood {t=b-l) ; and it has been shown by 

 one of us that the viscositr of the surface explains whr a solution of soap does 

 not spread on a solution of saponine or of albumen : and, on the other hand, the 

 hquid di'op being yiscous, there is no extension, or only a feeble one, since the 

 shght difference in tension is equihbrated by the resistance of the viscous Hquid. 



