MODERN PHYSICS 21 



crushing out the finest scientific brains in the world. Mose : 

 ley was killed at the age of twenty-seven, a year after hq 

 had made his epoch-making discovery, and all the lives, 

 and all the interests of the eternally infamous men who 

 made this war are not to be compared in value to the world 

 with a hair of Moseley's head. Yet he had to be sacri- 

 ficed to save a threatened civilization. A double honor to 

 Moseley. 



His discovery was this : He was analyzing the charac- 

 teristic X-rays which are given off when any kind of a 

 substance is bombarded with cathode rays. The experi- 

 ment was in my judgment as brilliantly conceived, as care- 

 fully and skillfully carried out, and as illuminating in its 

 results, as any which has been done in the last fifty years. 

 What he found was this, that the atoms of all the different 

 substances emit radiations or groups of radiations which 

 are extraordinarily similar, but that these radiations differ 

 in their wave lengths as we go from substance to sub- 

 stance. The whole discovery can be stated in this fashion : 

 If you take the highest frequency emitted by a given atom, 

 and if you lay down on a table a length which is equal to 

 the square root of this frequency, and if on top of that 

 you lay down the square root of the frequency of the 

 atom which has the next lower frequency, and so if you 

 continue to lay down, with one group of ends together, 

 the measured square root frequencies of all the elements 

 that you can study, then what have you got? You find 

 that you have a flight of stairs, with perfectly definite 

 equal treads; that is, the frequencies change by definite 

 steps as you go from element to element. And there are 

 only four vacant treads between the lightest element which 

 Moseley could study, namely aluminum and the heaviest 

 one, namely lead ; thus indicating that there are only four 

 elements in this range which we have not already found. 



We may then picture with considerable confidence this 

 whole physical world as built up out of one positive and 

 one negative electron. The positive electron is the nu- 



