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III. The Sheading of Fluids on Glass. 

 By W. B. Hardy*. 



(From the Goldsmith Metallurgical Laboratory, Cambridge.) 



THE phenomena which accompany the spreading of an 

 immiscible fluid over the surface of water under the 

 influence of surface forces have been known for a long time 

 and studied in great detail. The salient facts are these — 

 when a small drop of some, not all, fluids is placed on a clean 

 surface of water on which a few dust particles rest, a film of 

 insensible thickness rapidly spreads from it in all directions, 

 driving the dust particles before it. Sometimes, indeed 

 usually, the drop itself also expands to a plate of sensible 

 thickness which is in tensile equilibrium with the invisible 

 film. This plate usually is not in equilibrium. It thins in 

 places and, after certain changes, settles down to an irregular 

 or regular pattern, the spnces of which are occupied by an 

 invisible film, the counterpart of that which surrounds the 

 expanded drop. 



There are thus two distinct processes, the spreading to form 

 the invisible film, and the spreading of the drop to form a layer. 

 Some confusion has crept into the literature of the subject by 

 the use of the same word to describe these two dissimilar 

 processes. I propose therefore to distinguish the first as 

 primary spreading and the surface formed by it as a "pri- 

 mary composite surface," and the second as secondary 

 spreading which forms a "secondary composite surface." 

 A primary surface is covered by a film of the order of 

 1 micron in thickness, whilst a secondary surface is covered 

 by a layer from 50 to 500 microns in depth. 



By r employing pure chemical substances I was able to 

 verity Lord Rayleigh's suggestion, that the complicated 

 secondary changes in a secondary composite surface are due 

 to the chemical heterogeneity of the surface layer, and to 

 prove further that when a drop of a rigorously pure chemical 

 substance is placed on a clean surface of water, a primary 

 composite surface is formed by the spreading from it of an 

 invisible film, and there the matter ends, since the equilibrium 

 state for a single pure chemical substance spread on water is 

 such a surface in tensile equilibrium with a lens-shaped drop 

 into which the excess of the substance is gathered |. 



The existence of the invisible film is made manifest by the 



* Communicated by the Author. 



t Proc. Roy. Soc. A. lxxxviii. p. ol4 (1913). 



Phil. Mag. S. 6. Vol. 38. No. 223. July 1919. E 



