1917] on Soap Bubbles of Long Duration "iol 



Thus tile rate of lo<:"arithniic increase of the thickness is twice the 

 rate of logarithmic diniinntion of diameter. 



In hlack laibbles increase of thickness can he observed if the 

 contraction is rapidly effected b_v opening- the closed bulible to the 

 air, or by cautiously removing the air ])y suction ; but the immediate 

 effect is not to pioduce a coloured bubble. What happens is that small 

 silvery circular discs (seep. 8) l)reak out over the surface of the black 

 bubble, and these represent the aggregations of liquid resulting from 

 the contraction, for they are much thicker than the black film through 

 which they move. They very soon settle to the lower parts, and 

 there slowly coalesce to a small graded coloured area. . This usually 

 contracts in a short time to a much smaller zone, too thick to show 

 colour, and finally very little more is left than a small drop surrounded 

 M ith a narrow silvery band on the otherwise 1 )lack bubble. Large black 

 bid)bles contracting l)y gas percolation go too slowly for these effects 

 to l)e observable, but the final result is the same : a drop of liquid 

 slowly collects on the black bubl»le, and may in large bubbles grow 

 sufficiently to fall off, after causing a progressive elliptical distortion. 



Comparable behaviour is shown when one or two drops of solution 

 per minute are allowed to trickle into a black or partly black bubble. 

 Coloured streaks appear and pass downwards from places all round 

 the supporting nozzle into which the liquid is fed ; later on the streaks 

 break up into coloured discs, falling slowly by sinuous paths and with 

 diverse motions. However, they all pass through the black portion, 

 and after remaining for several minutes as a mass of distinct many- 

 coloured particles, form by coalescence a coloured zone below round 

 the drop. If the liquid is fed in more rajjidly, the coloured zone will 

 rise to a certain extent, but, in single bul)bles, a large area of black 

 almost always remains. The feeding of the liquid may be cautiously 

 increased almost to a stream ; the bubble is then, of course, much 

 disturbed at the Ijoundary of the black zone, as well as elongated by 

 tlie loading, and drops fall away in steady succession. The paths 

 through the black zone may then become so thick as to be transparent 

 threads of hquid, without any colour in themselves, but causing 

 coloured rays and patches to flash outwards from their paths. 



It was further observed that when a partly coloured bubble quickly 

 altered in size, the black zone remained roughly constant in area, being 

 independent of the extended or contracted surface. The thickness 

 f-eemed to alter at the expense of the coloured portion, which con- 

 sequently changed in tint. A bubble is allowed to develop to 

 blackness for about one-third of its surface, the remaining" two-thirds 

 grading from a silvery boundary at the black, through golden colour 

 to deep steel-blue This state is reached in a few hours with a 

 solution composed approximately of '^^ per cent, neutral ammonium 

 oleate in 83 per cent, glycerine, with a slight excess of ammonia. 

 Xow gradually expand the bubble by a slight reduction of the external 

 j'ressure. Meanwhile the level of ihe black boundary as well as the 



