366 Lord Rayleigh on Experiments 



Even in the interior parts of the surface at a distance from 

 the edge the sulphur particles do not retain their initial 

 positions, but form aggregations into which continually 

 increasing numbers are attracted. This is also due to gravity, 

 neighbours tending, as it were, to fall into the depression by 

 which every particle is surrounded. 



Camphor Movements a Test of Surface- Tension. 



The theory of these movements, due to Van der Mens- 

 brugghe, implies that they will take place with greater or less 

 vigour so long as the tension of the surface, which may be in 

 some degree contaminated, is greater than that of a saturated 

 solution of camphor. If, however, the contamination be so 

 great that the tension falls below this point, the solution of 

 camphor can no longer spread upon the surface, and the 

 movements cease. Thus, according to this theory and to 

 observations* upon a saturated solution of camphor, the 

 movements are an indication that the actual tension does not 

 fall below "71 of that of pure water. 



Although there appeared to be no reason for distrusting 

 this view, it was thought desirable to examine specially 

 whether the cessation of the movements was really a question 

 of surface-tension onlv, without regard to the character of 

 the contamination. The readiest method of ensuring the 

 equality of the tensions of two surfaces contaminated with 

 different materials is to make the two surfaces parts of one 

 surface, for two parts of the same surface cannot be at rest 

 unless they have the same tension. The method of experi- 

 ment was therefore to divide a surface of clean water con- 

 tained in a large dish into two parts bv a line of dust, and to 

 communicate different kinds of grease to the surfaces on the 

 two sides of the indicating line. If, for example, a small 

 chip of wood, slightly greased with olive-oil, be allowed to 

 touch one part of the surface, the line of dust is repelled by 

 the expansion of that part, but the effect may be compensated 

 by a slight greasino- of the other part with oil of cassia. By 

 careful alternate additions the line of dust may be kept central, 

 while the two halves become increasingly greased with the 

 two kinds of oil. At every stage of this process, so long as 

 the surface is at rest, the tension of all parts is necessarily the 

 same. 



A large number of substances have thus been tried in 

 pairs, of which may be mentioned oils of olive, cassia, turpen- 

 tine, lavender, cinnamon, anise, petroleum, pseudocumene. 



* Phil. Mag. November 1890. 



