548 SMITH'S INTERMEDIATE CHEMISTRY 



ments of the protons and electrons of which it is constructed (see 

 p. 195). Building up a more complex atom from its disintegration 

 products would require just as much energy as is evolved in the 

 disintegration. This is another step which remains for the 

 future. 



We may now proceed to examine more intimately the question 

 of atomic structure, already discussed in brief in earlier chapters 

 (pp. 195, 217). 



Atomic Numbers. Visible light, X-rays, and wireless elec- 

 tric waves are all vibrations of the same nature hi the ether. 

 They differ only in wave-length, the order of the wave-lengths 

 being 10~ 5 cm., 10~ 8 cm., and 10 6 cm. (10 kilometers), respectively. 

 Now, just as the spectrum of visible light is obtained by using a 

 grating, on which the rulings are separated by distances of the 

 order of the wave-length of such light, so ordinary crystals give 

 spectra of X-rays, because they are composed of particles arranged 

 in rows about one thousand times closer and so form a suitable 



grating for X-rays. This fact 

 was first discovered by Dr. 

 Laue of the University of 

 Zurich (1912). The X-rays 

 are produced in an evacuated 



.TIG. 121 



tube by cathode rays, which 



are streams of electrons emanating from the cathode (C, Fig. 

 121), when they strike the anticathode (A). 



With different elements on the anti-cathode, X-rays of slightly 

 different wave-lengths, and therefore giving different X-ray 

 spectra, are produced. The greater the number of free protons 

 (unit positive charges) in the nucleus of the atom, the shorter 

 should be the wave-length of the characteristic X-rays. It was 

 shown by Moseley (a brilliant young English physicist, killed at 

 Gallipoli) that when the elements are arranged in the order of 

 these wave-lengths, whole numbers can be assigned to each which 



