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XL. Experiments upon Surface-Films. 

 By Lord Rayleigh, Sec. R.S* 



THE experiments here described are rather miscellaneous 

 in character, but seem of sufficient interest to be worthy 

 of record. The greater number of them have been exhibited 

 in the course of lectures at the Royal Institution. 



The Behaviour of Clean Mercury. 



According to Marangoni's rule, water, which has the lower 

 surface-tension, should spread upon the surface of mercury ; 

 whereas the universal experience of the laboratory is that 

 drops of water standing upon mercury retain their compact 

 form without the least tendency to spread. To Quincke belongs 

 the credit of dissipating the apparent exception. He found 

 that mercury specially prepared behaves quite differently from 

 ordinary mercury, and that a drop of water deposited there- 

 on spreads over the whole surface. The ordinary behaviour 

 is evidently the result of a film of grease, which adheres with 

 great obstinacy. 



The process described by Quincke is somewhat elaborate ; 

 but my experience with water suggested that success might 

 not be so difficult, if only the mistake were avoided of pouring 

 the liquid to be tried from an ordinary bottle. In the early 

 experiments upon the camphor movements difficulty seems to 

 have been experienced in securing sufficiently clean water 

 surfaces. The explanation is probably to be found in the 

 desire to use distilled water, and to the fact that the liquid 

 would usually be simply poured from a stock bottle into the 

 experimental vessel. No worse procedure could be devised ; 

 for the free surface in the bottle is almost sure to be dirty, 

 and is transferred in great part to the vessel. In my expe- 

 rience water from the dirtiest cistern will exhibit the camphor 

 movements, provided that it be drawn in the usual manner 

 from a tap, and that the precaution be taken to give the vessel 

 a preliminary rinsing. 



In order to carry out the idea of drawing the liquid from 

 underneath, an arrangement was provided like an ordinary 

 wash-bottle, and was tilled with tolerably clean mercury. As 

 experimental vessels watch-glasses are convenient. They 

 may be dipped into strong sulphuric acid, rinsed in distilled 

 water, and dried over a Bunsen flame. When the glasses are 

 cool they may be charged with mercury, of which the first 



* Communicated by the Author. 



