Liquids on the Surface of Water. 45 



such a surface maintains the form of a spherical segment not 

 greatly differing from a hemisphere; but the water has very 

 little adhesion for mercury, so that the tension t ,f of the surface 

 of separation of the two bodies has a value very nearly that of 

 the tension t of the mercury. On the other hand, a and ct" have 

 no value, on account of the great density of the metal. The 

 equation therefore is only possible when the cos «' is very small — 

 that is to say, when a! scarcely differs from a right angle and 

 the drop is sensibly a hemisphere. The same equation shows 

 that when by any means we diminish /' or t n , the spreading im- 

 mediately follows. t ! may be diminished by touching the sur- 

 face of the segment with a morsel of soap; the soap -water 

 covers the free surface of the small liquid mass, and in a few 

 seconds the mass itself spreads, since soap-water has a much 

 smaller tension than pure water. The same result may be ob- 

 tained by touching the surface of the segment with a speck of oil. 

 28. I observed many years ago that a film of very dilute sul- 

 phuric acid on the surface of mercury was gathered up into a lens 

 by the action of the vapour of absolute alcohol, or of ether, held 

 over it. If a thin stratum of water adhere to a clean glass plate, 

 the vapour of ether or of alcohol will make it fly open, as already 

 noticed (10) . Creosote by contact will do the same. These appa- 

 rent repulsions between two liquids, of which so many examples 

 have long been known, are simple effects of surface-tension. It is 

 remarkable that so long ago as the year 1855 Professor Thomson 

 of Belfast 19 , brought before the British Association at Glasgow 

 the phenomena of " weeping," or " tears in the wine-glass," 

 and connected them with surface-tension. He says : " The more 

 watery portions of the entire surface having more tension than 

 those which are more alcoholic, drag the latter briskly away, 

 sometimes even so as to form a horizontal ring of liquor high up 

 round the interior of the vessel, and thicker than that by which 

 the interior of the vessel was wetted. Then the tendency is 

 for various parts of this ring or line to run together to those 

 parts which happen to be most watery, and so there is no stable 

 equilibrium for the parts to which the various portions of the 

 liquid aggregate themselves, soon to be too heavy to be sustained, 

 and so they fall down. In a wine-glass the thin film adhering 

 to the glass must very quickly become more watery than the 

 rest, by the evaporation of the alcohol being more rapid than 

 that of the water." Thus if a phial partly filled with wine d 

 corked be shaken and then left to rest, no such motions are ob- 

 served ; but if air be withdrawn by means of a tube, and fresh 



19 Phil. Mag. S. 4. vol. x. p. 330, " On certain curious Motions obser- 

 vable at the Surface of Wine and other Alcoholic Liquids." See also 

 note **. 



