upon Surface- Films* 365 



from spreading over the surface, as according to its lower 

 tension it ought to do, it remains suspended in the form of a 

 lens. And dust which may be lying upon the surface is not 

 driven away to the edge upon the deposit of the drop, as 

 would happen in the case of oil. A simple modification of the 

 experiment suffices, however, to clear up the difficulty. If 

 after the deposit of the drop a little lycopodium be scattered 

 over the surface, it is seen that a circular space surrounding 

 the drop, of perhaps the size of a shilling, remains bare, and 

 this however often the dusting be repeated, as long as any of 

 the CS 2 remains. The interpretation can hardly be doubtful. 

 The bisulphide is really spreading all the while, but on account 

 of its volatility is unable to reach any considerable distance. 

 Immediately surrounding the drop there is a film moving 

 outwards at a high speed, and this carries away almost instan- 

 taneously any dust that may fall upon it. The phenomenon 

 above described requires that the water surface be clean. If 

 a very little grease be present there is no outward flow, and 

 dust remains undisturbed in the immediate neighbourhood of 

 the drop. With the aid of the vertical lantern, and a shallow 

 dish whose bottom is formed of plai:e glass, these experiments 

 are easily shown to an audience. 



Movements of Dust. 



When dust of sulphur or lycopodium is scattered upon the 

 surface of water contained in a partially filled vessel, it is 

 found that after a few seconds the dust leaves the edge and 

 that a clear ring is formed of perhaps a centimetre in width. 

 Two explanations suggest themselves. The action may be 

 due to grease communicated to the surface from the edge of 

 the vessel ; or, secondly, it may be the effect of gravity upon 

 those particles of the dust which lie within the limits of the 

 capillary meniscus. The first explanation is rendered im- 

 probable by the non-progressive, or at least but very slowly 

 progressive, character of the effect ; and it is negatived by a 

 repetition of the experiment in a varied form. It is found 

 that if the vessel, whether of glass or metal, be filled over the 

 brim, so that the capillary meniscus is convex, then, although 

 as before a bare margin is formed, the effect is due to a motion 

 of the dust outwards (instead of inwards, as in the former 

 case), and therefore not to be attributed to grease. 



A similar movement of dust was to be observed in the 

 experiment above recorded, where magnesia was scattered 

 upon a pool of mercury, and is undoubtedly due to gravity ; 

 but the full explanation is not so simple as might appear at 

 first sight. 



Phil. Mag. S. 5. Vol. 33. No. 203. April 1892. 2 C 



