252 



NA TURE 



[July i6, 1891 



great difference as to the nature of the deviation from the 

 equilibrium condition, and as to the kind of force best 

 adapted to bring it about. The one gives way by becoming 

 varicose ; the other by becoming sinuous. The only 

 forces capable of producing varicosity are symmetrical 

 forces, which act alike all round. To produce sinuosity, 

 we want exactly the reverse— a force which acts upon the 

 jet transversely and unsymmetrically. 



I will now pass on to another subject for instantaneous 

 photography — namely, the soap film. Everybody knows 

 that if you blow a soap bubble it will break — generally 

 before you wish. The process of breaking is exceedingly 

 rapid, and difficult to trace by the unaided eye. If we 

 can get a soap film on this ring, we will project it upon 

 the screen and then break it before your eyes, so as to 

 enable you to form your own impressions as to the 

 rapidity of the operation. For some time it has been my 

 ambition to photograph a soap bubble in the act of 

 breaking. I was prepared for difficulty, believing that the 

 time occupied was less than the twentieth of a second. 

 But it turns out to be a good deal less even than that. 

 Accordingly the subject is far more difficult to deal with 

 than are those jets of water or coloured liquids which one 



Fig. 5. Fig. 6. 



can photograph at any moment that the spark happens to 

 come. 



There is the film, seen by reflected light. One of the 

 first difficulties we have to contend with is that it is not 

 easy to break the film exactly when we wish. We will 

 drop a shot through it. The shot has gone through, as 

 you see, but it has not broken the film ; and when the 

 film is a thick one, you may drop a shot through almost 

 any number of times from a moderate height without pro- 

 ducing any effect. You would suppose that the shot in 

 going through would necessarily make a hole, and end the 

 life of the film. The shot goes through, however, without 

 making a hole. The operation can be traced, not very 

 well with a shot, but with a ball of cork stuck on the end 

 of a pin, and pushed through. A dry shot does not 

 readily break the film ; and as it was necessary for 

 our purpose to effect the rupture in a well-defined manner, 

 here was a difficulty which we had to overcome. We 

 found, after a few trials, that we could get over it by 

 wetting the shot with alcohol. 



We will try again with dry shot. Three shots have 

 gone through and nothing has happened. Now we will 

 try one wetted with alcohol, and I expect it will break the 

 film at once. There ! it has gone ! 



The apparatus for executing the photography of a 

 NO. 1 133, VOL. 44] 



breaking soap film will of necessity be more complicated 

 than before, because we have to time the spark exactly 

 with the breaking of the film. The device I have 

 used IS to drop two balls simultaneously, so that 

 one should determine the spark and the other 

 rupture the film. The most obvious plan was to 

 nang iron balls to two electro-magnets, and cause them 

 to drop by breaking the circuit, so that both were let go 

 at the same moment. The method was not quite a suc- 

 cess, however, because there was apt to be a little hesitation 

 in letting go the balls. So we adopted another plan. 

 The balls were not held by electro-magnetism, but by 

 springs (Fig. 8) pressing lateraUy, and these were pulled 

 off by electro-magnets. The proper moment for putting 

 down the key and so liberating the balls, is indicated by 

 the tap of the beam of an attracted disk electrometer as it 

 strikes against the upper stop. One falling ball deter- 

 mines the spark, by filling up most of the interval between 

 two fixed ones submitted to the necessary electric 

 pressure. Another ball, or rather shot, wetted with 

 alcohol, IS let go at the same moment, and breaks the film 

 on Its passage through it. By varying the distances 

 dropped through, the occurrence of one event may be 

 adjusted relatively to the other. The spark which passes 

 to the falling ball is, however, not the one which illumi- 

 nates the photographic plate. The latter occurs within 

 the lantern, and forms part of a circuit in connection 

 with the outer coatings of the Leyden jars,i the 



whole arrangement being similar to that adopted 

 by Prof. Lodge in his experiments upon alterna- 

 tive paths of discharge. Fig. 8 will give a general idea 

 of the disposition of the apparatus. [Several photo- 

 graphs of breaking films were shown upon the screen ; 

 one of these is reproduced in Fig. 7.] ^ 



This work proved more difficult than I had expected ; 

 and the evidence of our photographs supplies the explana- 

 tion — namely, that the rupture of the film is an extra- 

 ordinarily rapid operation. It was found that the whole 

 difference between being too early and too late was 

 represented by a displacement of the falling ball through 

 less than a diameter, viz. \ inch nearly. The drop which 

 we gave was about a foot. The speed of the ball would 

 thus be about 100 inches per second ; therefore the whole 

 difference between being too soon and too late is repre- 

 sented by yj(j second. Success is impossible, unless the 

 spark can be got to occur within the limits of this short 

 interval. 



Prof. Dewar has directed my attention to the fact that 

 Duprd, a good many years ago, calculated the speed of 

 rupture of a film. We know that the energy of the film 

 is in proportion to its area. When a film is partially 

 broken, some of the area is gone, and the corresponding 

 potential energy is expended in generating the velocity of 



^ In practice there were two sets of ihree jars each. 



^ The appearance of the breaking bubble, as seen under instantaneous 

 illumination, was first described by Marangoni and Stephanelli, Nnovo 

 Cimenio, 1873. 



