304. WHAT IS LIFE 



trability)." (Great hardness means short wave- 

 length, or high frequency; soft rays mean greater 

 wave-length, or lower frequency.) According to 

 Millikan, the hardest X-rays have a wave-length of 

 0.1 A. It has been shown by Barkla that every ele- 

 ment when made the anti-cathode in a discharge 

 tube, will emit its own characteristic X-rays. These 

 can be photographed by employing a suitable 

 spectrometer, and the photographic plate will show 

 lines that are characteristic for the element, and that 

 are analogous to the spectral lines in an ordinary 

 spectrum. The "hardness" of the X-rays increases 

 with increase in the atomic weight of the element. 

 Much brilliant work has been done in research upon 

 X-ray spectra. The work of Laue, who (in 1912) in- 

 troduced the use of the crystal grating, is a landmark 

 in the study of X-rays. The Braggs — Sir W H. Bragg 

 and his son W. L.Bragg — stand out for their spectrom- 

 eter and their determination of the wave-lengths 

 of the X-rays of various metals. Notable work has 

 been done by Siegbahn, De Broglie, W. Duane, 

 A. W. Hull, D. L. Webster and Harry Clark, and 

 many others. Henry Moseley's study of the wave- 

 lengths of the characteristic X-rays of most of the 

 elements, resulted in his classic demonstration (1912) 

 of the arithmetic progression in the natural series 

 of the elements. With the aid of the modified Bohr 

 theory of the atom, the spectral lines of the elements 

 are slowly being deciphered, A. Sommerfeld leading 

 in this extremely difficult research. 



