WHAT THIS WORLD IS MADE OF 



It would pass the limits of this essay to attempt 

 to describe in detail how this amazing result has 

 been reached. Lord Kelvin in his famous lecture 

 on "Atoms" recounts four separate methods of 

 calculating the size of these particles ; and there are 

 two or three more which have been developed 

 since that lecture was delivered, twenty years ago. 



One of the first grew out of the curious bending 

 which a ray of light undergoes as it passes through 

 glass. The wave-lengths of light may be measured 

 with extraordinary accuracy, and, in order to ac- 

 count for this bending, Cauchy tried to calculate 

 how large the particles of glass would have to be. 

 It is now known that his estimate was far too 

 high, but the method, perfected by later investi- 

 gators, has served to set an upper limit at least. 



Another path lies in the familiar fact that two 

 metals, like zinc and copper, merely placed in 

 contact, develop a very small quantity of elec- 

 tricity. This, too, can be accurately measured, 

 and the amount of heat it would produce, were 

 it used, say, for electric cooking, may be quickly 

 calculated. Lord Kelvin conceived a way of 

 estimating how far apart the molecules of the 

 metals would have to be in order that the elec- 

 tricity they generate, by contact, should not melt 

 them. Knowing the weight of metal he used, he 

 could calculate that they must be more than a 



