TRANSACTIONS OF SECTION A. 525 



sharp tend of a stream, for instance, to anyone fishing. The more rapid flow into 

 the pool causes ascending currents, which, spreading out at the surface, give rise to 

 radial currents of pure water, which sweep back and hold at bay the oil or transparent 

 «cum on the surface of the rest of the pool, and which but for the outward motion 

 would rapidly extend over the pure surface. Under these circumstances the edge 

 of the scum is definitely marked by a fine rib, which shows itself in certain lights 

 Hs though a tine gut-line were floating on the water and were carried, first in one 

 direction and then in the other, according as the radial current or the spreading 

 force of the scum are in the ascendant. It is difficult to render this rib apparent 

 •on the surface of water contained in a vessel, although this may be one or two feet 

 in diameter. This may be done, but the motion which gives rise to the rib may be 

 rendered apparent by other means, — by dusting the surface of the pure water with 

 some insoluble powder, such as flowers of sulphur. The motion of the surface is 

 rendered apparent by the motion of the dust. It is then seen that the dust does not 

 fall back before the oil as though the surface of the pure water were in a general state 

 of contraction, for there is absolutely no motion in the dust except in the immediate 

 neighbourhood of the edge of the oil. It is as though the dust were swept back by 

 the advancing edge of the oil ; the dust, already swept up into a compact mass, 

 coming up to each fresh particle, pushes it before it, until a bright yellow band is 

 formed marking the edge of the oil. The residt is to give the impression that the 

 dust is being driven back by the oil— as if the oil were spreading fi-om some 

 inherent expansive force ; but, as a matter of fact, the oil is being drawn forward by 

 the contraction of the dust-covered surface of the pure water, and the fact that the 

 dust does not move till the oil reaches it shows that the contraction takes place 

 entirely at the edge of the oil in an almost infinitely narrow band. 



This phenomenon of surface-contraction is very remarkable, for it would be 

 inferred from other hydrodynamical phenomena that viscosity would to some 

 •extent resist the action of contraction, and thus tend to distribute this action eveix' 

 a considerable area, and that the contraction is not so distributed shows that there 

 is virtually no resistance to contraction, or that the surface-tension at the points at 

 which the surface is contracting is at least equal to the tension at those points of the 

 •surface which are at rest. 



This conclusion implies much more than the tacit assumption, made by Laplace 

 and subsequent writers, that the forces of cohesion obey the law of statical fluid 

 pressure — equality in all directions. It is weU known as regards other phenomena 

 that this law holds only when fluids are at rest or in uniform motion ; whereas here 

 we have a case in which the same law holds for a portion of fluid which is mo^-ing 

 with great rapidity relative to the fluid in its immediate neighbourhood. 



Laplace's theory is founded on an assumed attraction, between the molecules, 

 ■which attraction does not extend to sensible distances, and on the tacit assumption 

 ■already mentioned, that the pressure, whether impressed or molecular, is equal in 

 all directions. To explain the apparent absence of viscosity in the dynamical 

 phenomena some further assumptions are necessary. If the force of cohesion is due 

 to molecular attraction tliese dynamical phenomena require that the molecules 

 under their mutual attractions should not be in a state of equilibrium, except 

 in so far as they are held by the forces transmitted from one part of the fluid to 

 another. 



Such a condition would exist if the range of attraction extended bevond the 

 distance of a single molecule, that is, if the molecules are spherical or in such a 

 state of motion that they cannot fit like bricks. But whatever might be the shape 

 of the molecides, if the forces of cohesion acted between adjacent molecules only, 

 then they would be in equilibrium in all positions ; there would be no instability 

 and no rapid contraction, although, according to Laplace's theory, the force 

 would be sufficient to prevent extension of the surface, and hence to explain the 

 statical phenomena of capillary tension such as the suspension of drops. It is 

 therefore argued that these dynamical phenomena are important, as throwin"- a 

 •certain amount of liglit on the character of the forces which cause cohesion 

 between molecules. 



