364< New Experiments with Soap Bubbles. 



which a definite colour is visible, and the refractive index of 

 the solution be known. But this method does not readily 

 apply to our present inquiry, and would only be fairly appli- 

 cable to thicknesses producing the circles of colour produced 

 at the upper part of the bubble, which are described above, 

 since the same colour may be produced by various thicknesses 

 of film. 



Again, a slight examination of the bubble shows that 

 the colours vary considerably even in spaces only a line square, 

 especially when it is well expanded. This circumstance renders 

 the determination of the mean thickness of the bubble by the 

 optical method practically useless. It is, therefore, necessary 

 to have recourse to some other indirect means of weighing and 

 measuring. It is true we cannot put our bubble into the 

 balance, but still there is a device by which we can make a 

 determination so readily that any reader can make a direct 

 experiment on the thickness of the film of any particular 

 bubble. The method I refer to is to blow a bubble with a 

 gas of known specific gravity, which is less than that of air, 

 and of such a size that it will neither ascend or sink in the air, 

 whose specific gravity is known. The size of the balloon 

 being known, the weight that its contained gas will support is 

 readily calculated, and this gives the weight of the balloon. 

 This being known, the thickness of a spherical shell which 

 such a weight of liquid would form of the diameter of the 

 balloon is readily determined by a calculation, and this is equal 

 to the mean thickness of the balloon. From such experiments 

 the writer has found that bubbles have a thickness varying 

 from 1 5 , o o to 3 looo °f an inch*, the latter number re- 

 presenting that of a thin bubble. Wonderfully thin as is their 

 dimension, it is greatly exceeded by that of gold leaf, which 



* The following illustrates the method : — 



A bubble, carefully freed from drops of liquid at its lower pole, and of 35 in. 



diameter, filled with a mixture of one volume of hydrogen to sixteen volumes of 



air, showed no tendency to ascend or sink. 



Let s z= sp. gr. of hydrogen =. 00691. 



s'= sp. gr. of the bubble solution = 1*1. 



g ■=. number of grains in a cubic inch of water = 252'456. 



t = thickness of film . 



r = radius of balloon = 1*75 inches. 



w weight of balloon. 



m , /4r 3 ir 31 \ /4»- 3 ir 31 16 + s\ 4r 5 7r 



Then (-— x _) _ (_ x m x -^-) = -3- x 



31 1— s 



100 17 



w 



Also 4> ,2 7r I g s' = w very nearly. 



Whence equating the two values of w, and reducing, wo obtain 



JUL s whence substituting values t — in. 



3gs' 100 17 28124 



