of a Liquid Mass without Weight. 41 



Under the best circumstances, films of the liquid prepared as 

 above possess an extraordinary degree of permanence : a bubble of 

 1 decimetre diameter, deposited on a ring of iron wire 4 centims. in 

 diameter which has been previously wetted with the same solution, 

 may last for eighteen hours when freely exposed to the air of the 

 room — that is to say, six times as long as a bubble made with 

 the solution prepared as described in my Fifth Series. The sub- 

 stances used in making the liquid being manufacturing products, 

 vary in quality more or less in different specimens, so that I 

 have obtained a result like that just mentioned only exceptionally; 

 from what follows, however, an opinion may be formed of the 

 superiority of my new process, and of the degree of reliance which 

 may be placed upon it. 



Out of twenty-one successive preparations made during the 

 summers of the four years last past, and with different specimens 

 of Price's glycerine and of Marseilles soap, only two, in which 

 the soap and the glycerine employed were the same, were unsuc- 

 cessful ; but I have some reasons for suspecting an error in the 

 weighing of the soap. All these liquids were tested by means 

 of a bubble of 1 decimetre in diameter deposited on a ring, as I 

 have already said. Deducting the two cases of failure, nineteen 

 specimens remain : for three of these the longest time that a 

 bubble lasted was five hours ; for three others it was seven hours; 

 for two it was eight hours ; for four it was nine hours ; for four 

 others, ten hours ; for one, eleven hours ; for one, twelve hours ; 

 and for one, eighteen hours. 



A very remarkable thing is, that, when a bubble lasts for a 

 tolerably long time, the film acquires after an hour or two sen- 

 sibly the same thickness over the whole extent of the bubble, 

 excepting, of course, the small part at the bottom intercepted 

 by the metallic ring. This uniformity of thickness is apparent 

 from the disposition of the interference-tints. 



Another thing which is not of less interest is, that these tints 

 advance at first towards those of the first order, but afterwards 

 retrograde as far as the red or green of the last order, and 

 sometimes even as far as white. This degradation of the tints 

 arises, as I have shown in my Fifth Series, from the absorp- 

 tion of moisture from the surrounding air by the glycerine- 

 solution. 



The theory which I have formed of the glycerine- solution 

 leads to the further consequence that the substitution of pure 

 oleate of soda for Marseilles soap ought to yield, by a much 

 simpler process, a liquid far superior to even the best of those 

 prepared with soap. This anticipation is fully borne out by ex- 

 periment. In fact all that I found necessary was to dissolve the 

 oleate of soda in distilled water at a gentle heat, and then to 



