56 MORRIS LOEB 



of the distance between the earth and the sun, or of the great- 

 ness of the sun's diameter, we exclaim at the magnitude of the 

 figure, while we talk unconcernedly of molecules. Is it, how- 

 ever, so easy to realize the exceeding smallness of the mole- 

 cule? 



The measurement of the magnitude of molecules can be 

 performed in the following fashion. As a soap bubble expands 

 with its increasing inflation its film can be greatly stretched. 

 The thin bubble becomes more and more brilliant in color as 

 it grows thinner; and then a moment comes when the film 

 suddenly loses its color and becomes black. This is because a 

 certain thickness between the front and rear wall of a film is 

 required in order that the film shall show colors by reflection. 

 When it has become colorless, we know that we have stretched 

 it so that it no longer possesses this requisite thickness; it is 

 thinner than the length of a wave of light. In order that it 

 should be a continuous film, it must consist of at least one 

 row of molecules; therefore, a molecule is less thick than the 

 wave length of light is long. Immediately after the film is 

 stretched to this extreme thinness, it breaks, which is a proof 

 to us that there is not so very much difference between the 

 magnitude of the wave length of light and the molecule. 



Another interesting method of measuring the size of a mole- 

 cule is to be derived from the behavior of gases as they rub 

 upon one another, and while this involves much that could 

 not be studied here, it suffices to say that the same results are 

 obtained as by the previous method. Still other, but yet con- 

 cordant results can be obtained from a study of the expansion 

 of gases by heat, and their contraction under great pressure. 

 From all these we reach the conclusion that the molecules 

 have some real definite size, and likewise, that they consist of 

 comparatively few atoms apiece. 



In order that we may understand better what the rela- 



