620 Dr. F. W. Aston on the 



Spectrum I. on PL XV. shows the singly-charged lines of 

 neon, to the left of the C 2 group. It is reproduced here to 

 show the condition of the discharge-tube immediately before 

 compounds of chlorine were introduced. 



Chlorine (At.Wt. 35*46). 

 Spectra indicating that this element was a mixture of 

 isotopes were first obtained by the use of hydrochloric acid 

 gas, but as this was objectionable on account of its action 

 on mercury, phosgene (COCl 2 ) was substituted. Spectra II., 

 III., and IV. are reproduced from one of the plates taken 

 with this gas. It will be seen that chlorine is characterized 

 by the appearance of four very definite lines in the previously 

 unoccupied space to the right of 2 (32) : measurement shows 

 these lines to correspond exactly to masses 35, 36, 37, and 38. 

 There is no indication whatever of a line at a point corresponding 

 ivith the accepted atomic weight 35*46. On Spectrum II., taken 

 with a small magnetic field, faint lines will be seen at 17' 5 

 and 18*5. These only appeared when chlorine was intro- 

 duced, and are certainly second order lines corresponding to 

 35 and '61 . These figures seem to leave no possible escape 

 from the conclusion that chlorine is a mixture of isotopes and 

 that two of these have masses 35 and 37. It might be argued 

 that 36 and 38 are also elementary lines and at present there 

 is no evidence to deny this, but it is much more probable that 

 they are the hydrochloric acids HOP 5 and HC1 37 . The line 18 

 is no indication of an element 36, as it is doubtless due to OH 2 . 

 Corroborative evidence that CI 35 and CI 37 are the main i£ not 

 the only constituents is given bv the strong lines 63 and 65 

 (Spectrum IV.) probably due to COC1 35 and COC1 37 . If 

 chemical atomic weight is regarded as a statistical average, 

 any lines due to CI 35 or its compounds should be considerably 

 stronger than the corresponding ones due to CI 37 . This is 

 actually found to be the case. In all spectra taken with 

 chlorine present a faint line is distinguishable corresponding 

 to 39. It is just possible that this is a third isotope. 



The unquestionable accuracy of its combining weight on 

 the one hand and the striking whole-number masses given 

 on its mass-spectra by its individual particles on the other, 

 leave little doubt that chlorine is a mixed element, but much 

 critical work will be necessary before its constituents and 

 their relative proportions are decided with certainty. 



Argon (At.Wt. 39*88 Ramsay, 39*91 Leduc). 



At the close of the experiments with phosgene the discharge- 

 tube broke down and had to be cleaned and partially rebuilt, 



