PHYSICS — MILLIKAN. 181 



and with particular success by A. W. Hull at the laboratory of the 

 General Electric Co. There are almost unlimited possibilities for 

 the chemist in the discovery by this method of the exact positions of 

 the atoms in any kind of a crystal. 



But the results of this discovery, as of most of the others which I 

 have mentioned, are rather insignificant when compared with those of 

 the ninth which I am going to mention, namely the discovery of the 

 relations between the elements, and the extension of our knowledge 

 of the radiations emitted by different substances. This discovery 

 was made by a young Englishman only 26 years old, Moseley, who 

 has already, unfortunately, fallen a victim to this juggernaut which 

 is at the present time crushing out some of the finest scientific brains 

 in the world. Moseley was killed at the age of 27, a year after he 

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

 ests 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 sacrificed to save a threatened civilization. A double 

 honor to Moseley. 



His discovery was this: He was analyzing the characteristic^ 

 X rays which are given off when any kind of a substance is bom- 

 barded with cathode rays. The experiment was in my judgment as 

 brilliantly conceived, as carefully and skillfully carried out, and 

 as illuminating in its results as any which has been done in the last 

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

 substances emit radiations or groups of radiations which are extraor- 

 dinarily similar, but that these radiations differ in their wave lengths 

 as we go from substance to substance. 



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 con- 

 tinue 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. 



An extremely interesting question is, what is the greatest common 

 divisor of this series of steps, that is, what is the top step? There 

 are two ways to get at it. One is by filling all the spaces up to 



136650°— 20 13 



