Mr. J. Broughton on some Properties of Soap-bubbles. 229 



always in rapid motion. Many other appearances were observed; 

 among the most common are spots of such regularity that at 

 first sight they produce the effect of structure. Under a high 

 power, these latter are resolved into series of Newton's rings of 

 excessive minuteness. The variety of the phenomena is quite 

 remarkable; but the most commonly occurring effects are those 

 in which the colours red and green prevail. The motion ap- 

 pears to be invariable. It is diminished, though not prevented, 

 by first passing the light through a layer of water and covering 

 the bubble by a small glass shade. 



A flat film will also produce similar effects to a bubble when 

 viewed through the microscope, but the latter was found to 

 answer somewhat better. 



The determination of the thickness of the film in which such 

 effects are produced is a matter of some interest. Optical 

 methods are practically inapplicable, since various thicknesses 

 will produce the same colour, provided they fulfil the condition 



of being constantly either even or odd mul tiples of -• It was, 



however, attempted to determine the mean thickness by the fol- 

 - lowing method, and the results are probably approximately correct. 

 It consists in determining the weight of the bubble in the fol- 

 lowing manner: — A mixture of hydrogen and air was prepared 

 which, by filling a bubble, would cause it just to float in the 

 atmosphere without showing any considerable tendency to ascend 

 or descend. The bubble was carefully freed from an adhering 

 drop, and was measured by making it (while still attached to 

 the pipe) appear to coincide with the divisions of a measure when 

 viewed at a considerable distance. Such a determination gave 

 the following data : — 



A mixture of 1 volume of hydrogen and 16 volumes of air 

 caused a bubble of 90 millims. diameter to float. 

 Let r =" radius of bubble = 45 millims ; 



c = weight of 1 cubic centimetre of atmospheric air 



= •00129318 grm.; 

 h as density of hydrogen ='0591; 

 s = spec. grav. of bubble-solution =1*1; 

 w— weight of bubble; 

 t = thickness of film ; 

 then 



4r 3 7r 4rWl6 + A \ 



* So small is the ratio of the weight of a bubble to its volume, that they 

 can be made to ascend by dexterously filling them with warm breath. 



Phil Mag. S. 4. Vol. 31. No. 208. March 1866. R 



