1921] on Isotopes and Atomic Weights 305 



angle is selected by the diaphragm D and passed between the 

 circular poles of a powerful electromagnet 0, the field of which is 

 such as to bend the rays back again through an angle <f> more than 

 twice as great as 6. The result of this is that rays having a constant 

 mass (or more correctly constant m/ e ) w ^ converge to a focus F, 

 and if a photographic plate is placed at OF as indicated, a spectrum 

 dependent on mass alone will be obtained. On account of its analogy 

 to optical apparatus, the instrument has been called a positive ray 

 spectrograph and the spectrum produced a mass-spectrum. 



Fig. 2 shows a number of typical mass-spectra obtained by this 

 means. The numbers above the lines indicate the masses they 

 correspond to o:i the scale = 16. It will be noticed that the dis- 

 placement to the right with increasing mass is roughly linear. The 

 measurements of mass made are not absolute, but relative to lines 

 which correspond to known masses. Such lines due to hydrogen, 

 carbon, oxygen and their compounds are generally present as impuri- 

 ties or purposely added, for pure gases are not suitable for the 

 smooth working of the discharge tube. The two principal groups 

 of these reference lines are the C x group due to C(12), OH (13), 

 CH 2 (14), CH 3 (15), CH 4 or 0(16),"and the C 2 group (24 to 30) 

 containing the very strong line C 2 H 4 or CO (28). These groups will 

 be seen in several of the spectra reproduced, and they give, with the 

 C0 2 line (44), a very good scale of reference. 



It must be remembered that the ratio of mass to charge is the 

 real quantity measured by the position of the lines. Many of the 

 particles are capable of carrying more than one charge. A particle 

 carrying two charges will appear as having half its real mass, one 

 carrying three charges as if its mass was one-third, and so on. Lines 

 due to these are called lines of the second and third order. Lines 

 of high order are particularly valuable in extending our scale of 

 reference. 



When neon was introduced into the apparatus four new lines 

 made their appearance at 10, 11, 20 and 22. The first pair are second 

 order lines and are fainter than the other two. All four are well 

 placed for direct comparison with the standard lines, and a series of 

 consistent measurements showed that to within about one part in a 

 thousand the atomic weights of the isotopes composing neon are 

 20 and 22 respectively. Ten per cent of the latter would bring 

 the mean atomic weight to the accepted value of 20*20, and the 

 relative intensity of the lines agrees well with this proportion. 

 The isotopic constitution of neon seems therefore settled beyond all 

 doubt. 



The element chlorine was naturally the next to be analyzed, and 

 the explanation of its fractional atomic weight was obvious from the 

 first plate taken. Its mass spectrum is characterized by four strong 

 first order lines at 35, 36, 37, 38, with fainter ones at 39, 40. There 

 is no sign whatever of any line at 35 ■ 46. The simplest explanation of 



