OIL FILMS ON WATER AND MERCURl- — DEVAUX. 273 



deed these phenomena work spontaneously and are visible to you 

 all whenever a drop of grease falls upon the water in one of the 

 ordinary plates from which you eat, so that nothing is more common 

 and banal than these extremely thin films. 



The formation and stability of these films are automatic. The 

 stability is so great that it is possible, without breaking the film, 

 to distend it — that is, to separate progressively the molecules — until 

 their reciprocal action is entirely destroyed, an operation which we 

 could not perform upon liquids in bulk without leading immediately 

 to rupture. 



With our films, however, this is a most simple operation and always 

 successful; it is only necessary to increase the free surface occupied 

 by the film upon the water or the mercury. Thus we have become 

 acquainted with the fundamental fact that the extension of the oil 

 film is limited. As soon as the molecules are separated by a distance 

 greater by one to several tenths of their normal distance they lose 

 all reciprocal action, for they no longer diminish the surface tension ^ 

 of the water. We have called this phase the maximum extension. 

 Conversely it suffices to bring them together, by contracting the sur- 

 face slightly, in order to see the effect of the oil upon the surface 

 tension of the water reappear and increase rapidly, so that the ten- 

 sion passes rapidly from that of pure water to nearly that of oil. 



These facts allow us to enter directly the experimental study of 

 the field of molecular action.^ They allow us to catch a glimpse of 

 other mysteries to be discovered, other marvels to contemplate, and 

 to delve into that domain of invisible elements of which visible mat- 

 ter is composed. 



The little drop of oil has much more to show us. Who knows, 

 indeed, but that it will bring us before long phenomena of the great- 

 est importance, yet which at present we can not foresee ? 



1 A curious exception is found in oleic acid and in soap, the molecules of whicli wlien 

 stretched over water can be separated some tea times the molecular distance. Devaux, 

 1. c, 1904. 



= M. Brillouin so stated In the appreciation which this- professor of the College dc 

 France gave upon my researches at a meeting of the Soci4t6 de Physique on the 3d of 

 May, 1912. 



44863°— SM 1913 18 



