Liquids on the Surface of Water. 35 



years ago, but never could explain until I had read Professor Van 

 der Mensbrugghe's memoir. Prevost noticed that if a red-hot 

 iron wire or poker be presented obliquely to the end of a disk of 

 tinfoil floating on water, the disk will be repelled ; but if the hot 

 wire be held vertically over the disk, it does not move. By di- 

 recting the focus of a burning-glass on the disk, it is also re- 

 pelled. The water must be perfectly clean, or the experiment 

 will fail ; simply touching it with the finger is sufficient to pre- 

 vent the result 8 . 



I had fancied that the red-hot wire diminished the adhesion of 

 the surface locally, and during the expansion the disk got shifted 

 to a cooler part, where the adhesion was stronger; but this could 

 not apply to Macquer's observation, that gold spins when melted, 

 and that minute fragments of grit on its surface are powerfully 

 repelled. When the metal is fused by the action of a large lens, 

 fine powder on the surface is dispersed as if by a powerful blast 

 of the breath 9 . 



7. I proceed now to describe the method by which Professor 

 Van der Mensbrugghe gathers up the various facts described, 

 together with many others that will be indicated, and presents 

 them in harmonious relation to each other. 



8. Admitting the surface-tension of liquids to be a vera causa, 

 his first general proposition is as follows : — 



Let a and b be the respective surface-tensions of two liquids, 

 A and B, of which the second is supposed to be very volatile 

 and more or less soluble in the first, or, at least, capable of ad- 

 hering to it ; pour the liquid A into a large perfectly clean cap- 

 sule, then take up (by means of a capillary tube, for example) a 

 small quantity of the liquid B, and hold it vertically over and very 

 near to the surface of A ; then, if a be notably superior to b, the 

 subjacent layer will immediately move towards the edge of the 

 capsule and a circular depression will be produced, the more de- 

 cided in proportion as a exceeds b ; but if these quantities scarcely 

 differ from each other, there will be no movement on the sur- 

 face of A ; if, on the contrary, b is greater than a, there will be 

 developed a centripetal current ; that is to say, the part situate 

 beneath the tube will attract towards itself the neighbouring su- 

 perficial portions. 



9. In fact, if the vapour of B be more or less soluble in A, or 

 remain adherent to it, and if, moreover, a be very superior to b, 



8 Reference may also be made to P. Wilson's "Account of certain Mo- 

 tions which small lighted Wicks acquire when swimming in a basin of oil," 

 Edin. Trans, vol. iv. 



9 The surface-tension of metals in a state of fusion would form a good 

 subject of inquiry for any one who has ample means at his disposal. Re- 

 sults of great value might be expected from it. 



D2 



