1877.] 



Thickness of Soap Films. 



335 



which it is viewed, the position of the boundary of the black remains 

 fixed, though that of each of the other colours is altered with e^very 

 motion. More frequently, however, the presence of the colours of the 

 first order, and some or all of those of the second order, can only be de- 

 tected by means of a microscope, and to the naked eye several tints 

 appear to be wanting between the black and the colour which immediately 

 succeeds it. 



The constant recurrence of the phenomenon above mentioned, viz. the 

 very rapid change in the thickness of the film in the immediate neigh- 

 bourhood of the black, suggests an intimate connexion between the 

 thickness of the latter and the molecular constitution of the liquid. Any 

 investigation as to whether such a connexion exists must, it appears, be 

 commenced by seeking the answers to the following questions : — 



1. Do very rapid changes in thickness occur elsewhere in the films ? 



2. Is the black portion of the film uniform in thickness ? 



3. If so, is the thickness the same for all films formed with the same 

 liquid ? 



The authors have, with a single liquid and with the form of apparatus 

 employed, obtained results which, although not sufficiently numerous to 

 enable them to give general answers to these inquiries, display, as they 

 believe, a hitherto unsuspected constancy in the phenomena exhibited by 

 liquid films thinning under the influence of gravity, and seem to merit 

 further study. 



The method employed was to measure simultaneously the electrical 

 resistance of the films, and the breadths of the bands of colour they 

 displayed. 



The liquid used was M. Plateau's " Kquide glycerique," made by dis- 

 solving 1 part by weight of oleate of soda in 40 parts of water, and 

 adding 3 volumes of this liquid to 2*2 volumes of Price's glycerine. To 

 improve the conductivity, 3 parts by weight of potassium nitrate were 

 dissolved in every 100 parts of water along with the oleate of soda. 



The films which were submitted to investigation were cylindrical in 

 shape, and were formed between two platinum rings of the same diameter 

 placed one vertically over the other. The mode of supporting these and 

 making the electrical connexions was as follows : — A glass cylindrical 

 vessel, about 16 centims.high and 9 centims. in diameter, was fitted with 

 an ebonite cover divided into two unequal parts. Each of these would, if 

 placed on the top of the vessel, remain in situ. A brass tube, which 

 could be elevated or depressed, and was retained in its position by friction, 

 passed through the larger portion of the cover, and was, when in position, 

 in the centre of the glass vessel. A piece of india-rubber tubing provided 

 with a pinchcock was attached to the upper end of this tube, and to 

 the bottom of it was soldered a brass plate carrying the upper platinum 

 cylinder. This latter was formed of stout platinum foil, the edges of 

 which were welded together in order to avoid the introduction of any 



vol. xxvi. 2 A. 



