236 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



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 cor- 

 responding with the accepted atomic weight of 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 introduced, and 

 are certainly second-order lines corresponding to 35 and 37. 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 HC1 35 and 

 HO 37 . The line 18 is no indication of an element 36, as it is doubt- 

 less due to OH 2 . Corroborative evidence that CI 35 and CI 37 are the 

 main, if not the only, constituents is given by 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 one 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, so that by the 

 time it had reached suitable working conditions again, all traces of 

 chlorine had disappeared. The tube was run with a mixture of C0 2 

 and CH 4 , and then about 20 per cent of argon added. The main 

 constituent of the element was at once evident from a very strong 

 line at 40 (Spectrum VI), reproduced in the second and third orders 

 at 20 and 13.33 (Spectrum V). The third-order line is exceedingly 

 well placed for measurement, and from it the mass of the singly 

 charged atom is found to be 40.00 ±.02. At first this was thought to 

 be the only constituent, but later a faint companion was seen at 36, 

 which further spectra showed to bear a very definite intensity rela- 

 tion to the 40 line. No evidence drawn from multiple charges is 



