2,3^ 



NATURE 



[May 12, 1921 



and third order. Lines of high order are particu- 

 larly 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 measure- 

 ments showed that to within about one part in a 

 thousand the atomic weights of the isotopes com- 

 posing neon are 2000 and 2200 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. 



These rays are formed by a normal, positively 

 charged ray picking up two electrons. On the 

 negative spectrum of chlorine only two lines, 35 

 and 37, can be seen, so that the lines at 36 and 

 38 cannot be due to isotopes of the element. These 

 results, taken with many others which cannot be 

 stated here in detail, show that chlorine is a com- 

 plex element, and that its principal isotopes are 

 of atomic weight 35 and 37. There may be, in 

 addition, a small proportion of a third of weight 

 39, but this is ^doubtful. Spectra II., III., and 

 IV. show the results with chlorine taken with 

 different magnetic field strengths. 



The objection has been raised on many occa- 

 sions that if chlorine consists of isotopes, how is 

 it that its atomic weight has been determined so 



V 

 VI 



VII ~ 



vfifm 



'> i3 Ct 



C») « c«> 



\ I I I i 



"mk 



jm ^imtrng^^^m^^^^jf^^^Tm 



\CL 



it: ■ \S. 



-MEL 



w 



\l / 



* b 



\ I / 

 III 



\ I / 



I I 



00 o *^ ^^ ^ "^O 



I I \ I / I 



Kr 



Fig. 2. — Typical mass-spectra. 



The element chlorine was naturally the next to 

 be analysed, and the explanation of its fractional 

 atomic weight was obvious from the first plate 

 taken. Its mass-spectrum is characterised 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 

 the group is to suppose the lines 35 and 37 are 

 due to the isotopic chlorines, and lines 36 and 38 

 to their corresponding hydrochloric acids. The 

 elementary nature of lines 35 and 37 is also indi- 

 cated by the second-order lines at 17-5, 18-5, and 

 also, when phosgene was used, by the appearance 

 of lines at 63, 65, due to CC)CP« and COCF. ' 



Qliite recentty it has been found possible 'to 

 obtain tKe soectrurri of negatively' charged rays. 

 NO. 2689, VOL. 107] 



accurately and so consistently by different 

 chemists? The obvious explanation of this 

 appears to be that all the accurate determinations 

 have been done with chlorine derived originally 

 from the same source — the sea — which has been 

 perfectly mixed for aeons. If samples of the 

 element are obtained from some other original 

 source, it is quite possible that other values of 

 atomic weight will be determined, exactly as in 

 the case of lead. 



The mass-spectrum of argon shows an exceed- 

 ingly bright line at 40, with second-order line at 

 20, and third-order line at 13 J. The last is par- 

 ticularly well placed between known reference 

 lines, and its measurement showed that the triply 

 charged atom causmg it had a" mass 4o'oo very 



