and of certain Liquids on the Surface of Water. 383 



17. Now in all these cases I venture to say that the experi- 

 ment was not performed under the most favourable conditions. 

 It was known that the water must be free from grease; but I 

 know of no reference on the part of these early inquirers to 

 show that they took special means to make the vessel containing 

 the water chemically clean, any more than the surface of the 

 camphor or the knife with which it was cut or scraped. Hence, 

 the initial tension of the water not being at a maximum, contact 

 with a very slight portion of grease or oily matter would so far 

 further lower it as to produce the sudden effects described. Bat 

 if the experiment be repeated under the most favourable condi- 

 tions, in which the surface-tension of the water is at its maxi- 

 mum, the water may be touched with a fixed oil, and the iri- 

 descent film resulting therefrom, though lowering the tension, 

 will not do so to a sufficient amount to arrest the motions of the 

 camphor fragments. For example, a shallow glass 4 inches in 

 diameter was filled first with strong sulphuric acid and then 

 rinsed out with tap-water and filled with the same. Fragments 

 of camphor were extremely active on the surface, which was now 

 touched with the point of a clean penknife that had been dipped 

 into refined East-India rape-oil ; a film of a splendid deep-blue 

 colour was produced, which instantly opened into a sort of lace- 

 pattern ; but the fragments of camphor continued to rotate, not 

 so vigorously as at first, but still with vigour. If the film of 

 oil be first formed on the surface and the camphor fragments 

 thrown upon it, they will also rotate. If instead of using the 

 point of a knife to deposit the oil, a glass rod be employed and a 

 drop be deposited, this, especially if the oil be heated, will flash 

 out into a film, completely covering the surface of water in a 

 vessel 6 inches in diameter. Under these conditions fragments 

 of camphor do not rotate; but there is sufficient residual tensile 

 force to give motion to creosote. A drop of this instantly re- 

 pels the oil-film, cuts it up in all directions, and moves over the 

 surface with great vigour, the only sign of diminished tension 

 being that the cohesion-figure does not open out into the usual 

 brittle arc and so break up into a number of figures, but pre- 

 serves one parent figure, from which volleys of small globules are 

 discharged 27 . 



18. In the paper just referred to 28 I give a number of cases 

 in which a volatile-oil film arrests the motions of creosote, and 

 then evaporating, the water (according to the new theory) so far 

 recovers its tension that the creosote suddenly starts into life and 

 increases in vigour as evaporation proceeds. 



27 Figures representing this action are given in the plate accompanying 

 the paper referred to in note 23 . 



28 See note 23 . 



