On Equilibrium-Figures of a Liquid Mass. 287 



Besides, it is possible, without having recourse to mathematical 

 analysis, sufficiently to account for this identity. Let me repeat, 

 for this purpose, a principle on which I have many times dwelt in 

 the preceding series. When a surface fulfils the general condi- 

 tion of equilibrium, it is indifferent whether the liquid be on one 

 side or on the other of this surface ; in other words, to each 

 figure in a state of equilibrium, which is in relief, corresponds 

 a figure in a state of equilibrium, identical with the same, only 

 in depression. Now, the two faces of a liquid film, on account 

 of the thinness of the latter, being capable of being considered 

 as though they were two identical surfaces, the one in relief and 

 the other depressed with regard to the liquid which forms the 

 film, it follows, from the principle in question, that if one of these 

 two faces constitutes a surface of equilibrium, it is the same with 

 the other face, and that thus equilibrium exists throughout the 

 entire film. 



Suppose, however, it was possible to form in air liquid films 

 devoid of weight ; these films would necessarily take the same 

 form as the films of oil formed in the alcoholic mixture. Now 

 liquid films formed in air (films of soapy water, for example) 

 are so thin that the action of gravity upon them can generally 

 be regarded as inappreciable in comparison with -that of mole- 

 cular forces ; we should therefore obtain in air, with films of 

 soapy water or of an analogous liquid, the same figures in a 

 state of equilibrium as with films of oil in the alcoholic mixture, 

 and consequently, after what I have said above, figures which 

 would belong to a full liquid mass devoid of weight. Therein 

 consists the process which I have mentioned. 



Thus we arrive at the curious result, that, with a liquid acted 

 upon by gravity and in a state of repose, one can produce on a 

 large scale all forms of equilibrium which belong to a liquid 

 mass without weight and likewise at rest. 



Soap-bubbles offer the first example for the employment of 

 the process under consideration ; floating in air, they are sphe- 

 rical, just as a full liquid mass would be if devoid of weight and 

 freed from all adhesion. 



The films, however, which are obtained from common solution 

 of soap have but a very short existence, unless they be in a 

 close vessel ; a soap-bubble of one decimetre diameter, formed 

 in the open air of a room, rarely lasts two minutes; it was 

 therefore essential to find out some better liquid ; and I have 

 been happy enough to discover one which furnishes in the open 

 air, whilst preserving its liquid nature, films of great durability. 

 This liquid is formed by mixing, in proper proportions, glycerine, 

 water, and soap. A glycerine which seems very pure and very 

 concentrated can be easily procured at no great cost in London, 

 at Mr. Bolton's, 146 Holborn Bars, for instance. I shall point 



