SOAP-BUBBLES. 587 



little holes in the surface of the liquid ; and when a hole is made the 

 surface-tension tends to tear the liquid away, and to make it bigger. 

 If the liquid has a very considerable surface-tension, the small holes 

 in the surface may be so instantly turned into large ones that the 

 bubble may burst. This is, however, less likely to occur wdien the 

 surface-viscosity is small than when it is great, because in that case 

 the liquid flowing in from all sides can more easily fill up the hole, and 

 restore the damage done, before it becomes dangerously large. The 

 best kind of bubble for lasting is one in which the surf ace- viscosity is 

 tolerably large, so that the sides of the bubble may not become thin 

 too quickly, and in which the surface-tension is not too great, so that 

 any small fractures which occur may not be instantly enlarged. When 

 we find a liquid which has these two properties, we have all the requi- 

 sites for making good bubbles ; but sooner or later a hole is made, 

 and then the bubble bursts, and in a way which is probably very dif- 

 ferent from what, a priori, we should expect. In the first place, the 

 orifice which has been formed becomes rapidly larger, the surface- 

 tension which acts all round its edges and pulls the film away from 

 its centre tending to enlarge it. Secondly, the surface of the liquid is 

 necessarily very much curved all round the hole, and a greater press- 

 ure is therefore excited at that part by the surface on the liquid 

 which forms the interior of the film than elsewhere. Hence the liquid 

 becomes heaped up around the hole into a ring which is thicker than 

 the rest of the bubble, though its thickness is very small compared 

 with the diameter of the hole. The liquid iu the ring is thus in cir- 

 cumstances somewhat similar to that in the long cylinder we have 

 already studied it undergoes a similar series of transformations and 

 is broken up into drops which are flung away from the bubble. 

 Another ring is instantly formed and as instantly broken, and the 

 process is repeated again and again with inconceivable rapidity, until 

 in a very small fraction of a second a little cloud, composed of the 

 numerous minute drops which have been formed, is all that remains 

 of the bubble. 



I must now draw to a close. I have discussed with you, as well 

 as I could in the short space of time allotted to me, the history of a 

 bubble from its birth, in the bosom of the liquid, to its dissolution in 

 the air above. The facts and experiments I have brought to your 

 notice have been, I hope, in themselves sufficient to attract you ; but 

 I think they will acquire an additional interest if, before we part, I 

 tell you something about the man to whom we owe most of our knowl- 

 edge on the subject of my lecture. I mean M. Plateau, the Professor 

 of Physics in the Belgian University of Ghent. This gentleman began 

 his studies on liquids when a young man, and was already well known 

 for his success in scientific investigation, when a misfortune overtook 

 him which one would have thought would have put an end to his 

 further researches. He became hopelessly blind. A misfortune like 



