TENACITY OF SOAP FILMS. 257 



liquid. Dissolve 1 part of photographic guncottoii in a mixture 

 of 1 part absolute alcohol and 16 of ether, and allow the liquid to 

 subside if not quite clear ; decant off the clear solution and add to 

 it about three-fourths of its volume of pure castor oil ; extremely 

 tenacious films can be produced 

 from this fluid. 



Expt. 302. Tenacity of thin 

 Films. The great strength and 

 tenacity of soap films made from 

 a proper solution may be illus- 

 trated by the following experi- 

 ments. Blow a good-sized bubble 

 and make it adhere to a ring of 

 wire so as to rest on the ring 

 (Expt. 300) ; carefully bring to 

 the top of the bubble a circular 



bit of paper, through the centre Fig. 108. Soap Bubble supported on 

 of which a knotted thread or thin Wire Ring, and pulled upwards 

 string has been passed (fig. 108), into a conical shape by means of 

 the surface of the paper being 



wetted with soap liquor ; the paper will then adhere to the bubble ; 

 by pulling the thread the bubble can be elongated and pulled up- 

 wards to a considerable extent without breaking; conversely, if 

 the bubble be made to hang down from the ring, and the paper 

 disc and thread be attached below, and a little paper cage or tray 

 be tied to the thread, a number of small 

 shot can be put in the cage and their 

 weight suspended by reason of the ten- 

 acity of the film of soap liquor due to its 

 " surface tension " ; a light wire ring with 

 a cage suspended therefrom by three 

 threads (fig. 109), may be used instead 

 of the paper disc and knotted thread. 

 Exact numerical experiments made with 

 soap films show that the amount of weight 

 which is required to increase the area of -pig. 109. Soap Bubble sup- 

 the film to a given extent is directly pro- ported by Wire Ring 

 portionate to the increase in area thus an(1 pulled downwards 

 brought about. A soap film may thus be **&% 

 used like the spring balance used tor weigh- 

 ing letters, and similar objects, where the amount of extension or 

 compression of the spring used is proportionate to the weight acting ; 

 and where, consequently, the amount of weight can be read off on a 

 scale by means of an " index " or pointer attached to the spring. 



K 



