upon Surface-Films. . 369 



finger repeated, the effect was more pronounced than before, 

 and the bared space much larger, showing that the treatment 

 with the spirit-flame had driven away most of the residual 

 contamination. 



The best effects were obtained with a dish somewhat larger 

 than that above mentioned ; and in subsequent experiments 

 the difference of temperature between different parts was more 

 readily maintained by the use of a vessel in which the main 

 portions were connected by a comparatively narrow channel. 

 In this way the tensions of surfaces, contaminated in different 





Fig.l. 























degrees, may be equalized, the warmer purer surface in one 

 compartment balancing the colder but greasier surface in the 

 other. And the actual temperature difference necessary for 

 equilibrium gives a measure of the small difference of tensions 

 to be compensated*. 



When the surface of the liquid in the tin vessel is but very 

 slightly greased, a spot can no longer be cleared by the warmth 

 of the finger held underneath. Indeed the spirit-flame itself 

 soon becomes ineffective. And yet the greasing may be so 

 slight that camphor fragments move with apparently unabated 

 vigour. 



It is of interest to compare the behaviour of saturated 

 solution of camphor with that of greasy water. The former 

 can scarcely be brought to rest, unless covered up. This is 

 doubtless due to evaporation of camphor, aided by local 

 draughts. A spirit-flame drives away dust in a manner im- 

 possible in the case of a merely greasy surface, whose tension 

 may nevertheless be decidedly higher than that of the cam- 

 phorated water. 



It may here be mentioned that the lowering of tension by 

 camphor follows a different law from the lowering caused by 

 soap. In the latter case the fall of tension requires time, and 

 at the first moment of its formation a free surface has almost 

 the tension of pure water. Similar experiments to those 



* The lowering- of tension per degree Cent, is said to be *0018 of the 

 total value. 



