PLATEAU ON SOAP-BUBBLES. 397 



are not in continuous communication, the film is no longer the surface of absolute 

 minimum area, but the surface which, with the given boundary, and inclosing a 

 given volume, has a minimum area. 



M. Plateau has gone at great length into the interesting but difficult ques- 

 tion of the conditions of the persistence of liquid films. He shews that the 

 surface of certain liquids has a species of viscosity distinct from the interior 

 viscosity of the mass. This surface-viscosity is very remarkable in a solution of 

 saponine. There can be no doubt that a property of this kind plays an impor- 

 tant part in determining the persistence or collapse of liquid films. M. Plateau, 

 however, considers that one of the agents of destruction is the surface-tension, 

 and that the persistence mainly depends on the degree in which the surface- 

 viscosity counteracts the surface-tension. It is plain, however, that it is rather 

 the inequality of the surface-tension than the surface-tension itself which acts as 

 a destroying force. 



It has not yet been experimentally ascertained whether the tension varies 

 according to the thickness of the film. The variation of tension is certainly 

 insensible in those cases which have been observed. 



If, as the theory seems to indicate, the tension diminishes when the thick- 

 ness of the film diminishes, the film must be unstable, and its actual persistence 

 would be unaccountable. On the other hand, the theory has not as yet been 

 able to account for the tension increasing as the thickness diminishes. 



One of the most remarkable phenomena of liquid films is undoubtedly the 

 formation of the black spots, which were described in 1672 by Hooke, under 

 the name of holes. 



Fusinieri has given a very exact account of this phenomenon as he observed 

 it in a vertical film protected from currents of air. As the film becomes thinner, 

 owing to the gradual descent of the liquid of which it is formed, certain portions 

 become thinner than the rest, and begin to shew the colours of thin plates. 

 These little spots of colour immediately begin to ascend, dragging after them 

 a sort of train like the tail of a tadpole. These tadpoles, as Fusinieri calls 

 them, soon begin to accumulate near the top of the film, and to range them- 

 selves in horizontal bands according to their colours, those which have the 

 colour corresponding to the smallest thickness ascending highest. 



In this way the colours become arranged in horizontal bands in beautiful 

 gradation, exhibiting all the colours of Newton's scale. When the frame of the 

 film is made to oscillate, these bands oscillate like the strata formed by a 



