PHYSICS IN 1913 609 



merits which await confirmation are Aston's separation of a new 

 gas of atomic weight 22, very closely allied to neon, from this 

 gas, and Stark's splitting up of the hydrogen lines by the appli- 

 cation of an intense electric field, corresponding to the Zeeman 

 effect in a magnetic field. The effect of small traces of gas on 

 some of the electric properties of metals has been brought into 

 prominence, and the very existence of the photoelectric effect 

 for absolutely gas-free metals has been questioned. Several 

 other researches of importance are described in the following 

 pages, and we may say that the discoveries of the past year 

 have opened up more than one field of research which promises 

 rich results. 



The first experiments on the diffraction of X-rays by a 

 crystal were described in June 191 2 by Friedrich, Knipping, and 

 Laue in a paper entitled " Interference Phenomena with the 

 Roentgen Rays." In these experiments a fine pencil of rays 

 from an ordinary X-ray bulb was passed through a slip of 

 crystal and received on a photographic plate placed behind the 

 crystal at right angles to the incident beam. The plate on de- 

 velopment showed not only a very dark spot, corresponding to the 

 direct beam, but also a series of other fainter spots surrounding 

 it in a complicated geometrical pattern of high symmetry. 

 These spots are formed by beams of X-rays scattered in definite 

 directions from the crystal, and making, in some cases, angles as 

 large as 45 with the direct beam. Laue's theory, which led to 

 the experiments, was that the crystal, because of the regularity 

 of its structure, formed a natural diffraction grating, which 

 differed from the ordinary ruled grating firstly in having a very 

 much smaller grating space, and secondly in being an arrange- 

 ment in three dimensions, analogous to a set of plane gratings 

 placed one behind the other. Each molecule he supposed to be 

 capable of emitting secondary wavelets when struck by the 

 incident ray, and by assuming the molecules to be arranged in a 

 simple system, each one being at the corner of an elementary 

 cube, he accounted for the positions of the spots on the plate : 

 to explain the absence of other spots which might be expected 

 he had to assume that the incident pencil of X-rays did not 

 contain a continuous range of wave-lengths, but a certain five 

 wave-lengths which he calculated. The order of magnitude 

 found for them was from 1 to 4 times io -9 cms. This is about 

 the same order as the length previously suggested by Walter 



