180 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



the inside of the sphere, we can obtain the number of alpha particles 

 deflected through any given angle, and then with a little analysis 

 of unquestionable correctness, we can find how many unit charges, 

 positive electrons, there are in this exceedingly small nucleus, and 

 this number comes out approximately one-half of the atomic weight. 



Now, I come to another extraordinary discovery which did not 

 merely tell us approximate!} 7 how many electrons there are in the 

 nucleus, but it told us exactly how many there are, and the resu.lt 

 checked too with the number obtained by the foregoing approximate 

 method. This brings me to the recent discoveries in the field of 

 X ra}-s, and I will call the seventh of the modern advances the dis- 

 covery of the nature of X rays, which was virtually made by Barkla 

 in 1904; for Barkla and others had proved that there are two types 

 of X rays, first X rays which consist of simple ether pulses pushed 

 off from an electron when it changes its speed; and second so-called 

 characteristic X rays which are formed thus: When the electrons 

 bump into a target they set something in the target into vibration, 

 and this something sends off perfectly definite characteristic X rays, 

 which are like monochromatic light. So, we have two types of 

 X rajs, pulse X rays, like white light, and monochromatic X lays 

 like monochromatic light, such as comes from a mercury-vapor 

 lamp. That is the seventh of our great modern discoveries and it 

 must be credited chiefly to Barkla. 



The eighth I will call the discovery of crystal structure by the 

 study of X rays, which is due to Laue in Munich, and Bragg, in 

 England. The method is simply this. You know that we analyze 

 light by a grating which consists of a series of equally spaced lines 

 on a reflecting or transmitting surface. With such a device we can 

 split light up into a spectrum, but we can not thus split it up unless 

 the width of the grating space is comparable with the wave length of 

 the light. In the case of X rays, we had no knowledge of gratings 

 whose grating spaces were anything like as small as the wave length 

 of X rays, in fact such gratings were unknown until Laue had the 

 bright idea of using the regular arrangement of the atoms in a 

 crystal for a grating to see whether that would not do the work, and 

 it did the work marvelously well. It was found that we could com- 

 pute the grating space of certain crystals from the densitj' and the 

 atomic weight and then from the observed spectrum find the wave 

 length of the X rays used. And now knowing this wave length we can 

 work backward and find the grating-space for other crystals. We 

 are now using this method for finding the positions and the arrange- 

 ments of the atoms in crystalline bodies. Professor Bragg in his 

 recent book on X rays and crystal structure has described this work 

 very beautifully. Thus a whole new field of experimentation has 

 been opened up and is being pursued in a great many laboratories, 



