398 PLATEAU OX SOAP-BUBBLES. 



of liquids of different densities. This shews that the film is subject to 

 dynamical conditions similar to those of such a liquid system. The liquid is 

 subject to the condition that the volume of each portion of it is invariable, 

 and the motion arises from the fact that by the descent of the denser portions 

 (which is necessarily accompanied by the rise of the rarer portions) the gravi- 

 tational potential energy of the system is diminished. In the case of the film, 

 the condition which determines that the descent of the thicker portions shall 

 entail the rise of the thinner portions must be that each portion of the film 

 offers a special resistance to an increase or diminution of area. This resistance 

 probably forms a large part of the superficial viscosity investigated by M. PlaU 

 which retards the motion of his magnetic needle, and evidently is far gren 

 than the viscosity of figure, in virtue of which the film resists a shearing motion. 



The coloured bands gradually descend from the top of the film, presei 

 at first a continuous gradation of colour, but soon a remarkable black, or nearly 

 black, band begins to form at the top of the film, and gradually to extend 

 itself downwards. The lower boundary of this black band is sharply detii: 

 There is not a continuous gradation of colour according to the arrangement in 

 Newton's table, but the blaok appears in immediate contact with the wL 

 or even the yellow of the first order, and M. Fusinieri has even observed it 

 in contact with bands of the third order. 



Nothing can shew more distinctly that there is some remarkable change 

 in the physical properties of the film, when it is of a thickness somewhat greater 

 than that of the black portion. And in fact the black part of the film is in 

 many other respects different from the rest. It is easy, as Leidenfrost tells us, 

 to pass a solid point through the thicker part of the film, and to withdi; 

 it, without bursting the film, but if anything touches the black part, the film 

 is shattered at once. The black portion does not appear to possess the mobility 

 which is so apparent in the coloured parts. It behaves more like a brittle solid, 

 such as a Prince Rupert's drop, than a fluid. Its edges are often very irre^ii 

 and when the curvature of the film is made to vary, the black portions some- 

 times seem to resist the change, so that their surface has no longer the same 

 continuous curvature as the rest of the bubble. We have thus numerous indi- 

 cations of the great assistance which molecular science is likely to derive from 

 the study of liquid films of extreme tenuity. 



We have no time or inclination to discuss M. Plateau's work in a critical 

 spirit. The directions for making the experiments are very precise, and if some- 



