136 Lord Rayleigh. [June 5, 



question, may here be recorded. First, as to the effect of soap, or 

 rather oleate of soda. A surface of distilled water was prepared by 

 wind and heat until the time was 5, indicating a high degree of 

 purity. The door being closed, so as to isolate the two parts of the 

 surface, and the wind being maintained all the while, a few drops of 

 solution of oleate were added to the water in the tail-piece. With the 

 aid of gentle stirring, the oleate found its way, in a few minutes, 

 under the door, and reached the surface of the water in the well. 

 The time gradually rose to 13, 14, 15 ; and no subsequent treatment 

 with wind and heat would reduce it again below 12. In this case 

 there can be no doubt that the contamination comes from the interior, 

 and is quickly renewed if necessary ; not, however, so quickly that 

 the tension is constant in spite of extension or the surface would be 

 free from superficial viscosity. 



In like manner, the time upon the surface of camphorated distilled 

 water could not be reduced below 10, and the behaviour of motes 

 before the advancing needle was quite different to that observed 

 upon a clean surface. A nearly saturated solution of chloride of 

 sodium could not be freed from superficial viscosity ; while, on the 

 other hand, an addition of ^ per cent, of alcohol did not modify the 

 behaviour of distilled water. 



The films of grease that may be made evident in Plateau's ap- 

 paratus are attenuated in the highest degree. In a recent paper* I 

 have estimated the thickness of films of olive oil competent to check 

 the movements of camphor fragments as from one to two micro- 

 millimetres ; but these films are comparatively coarse. For example, 

 there was never any difficulty in obtaining from tap-water surfaces 

 upon which camphor was fully active without the aid of wind or 

 special arrangements. I was naturally desirous of instituting a com- 

 parison between the quantities necessary to check camphor move- 

 ments and the more minute ones which could be rendered manifest 

 by Plateau's needle; but the problem is of no ordinary difficulty. A 

 direct weighing of the contamination is out of the question, seeing 

 that the quantity of oil required in the well of the apparatus, even to 

 stop camphor, would be only -fa milligram. 



The method that I have employed depends upon the preparation of 

 an ethereal solution of olive oil, with which clean platinum surfaces are 

 contaminated. It may he applied in two ways. Either we may rely 

 upon the composition of the solution to calculate the weight of oil 

 remaining upon the platinum after evaporation of the solvent, or we 

 may determine the relative quantities of solution required to produce 

 the two sorts of effects. In the latter case we are independent of the 

 precise composition of the solution, and more especially of the 



* Supra, p. 364. 



