306 Mr. F. W. Aston [Feb. 11, 



the group is to suppose that the lines 85 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 indicated 

 by the second order lines at 17 * 5, IS* 5, and also, when phosgene 

 was used, by the appearance of lines at 63, 65, due to COCl 35 and 

 COCl 37 . 



Quite recently it has been found possible to obtain the spectrum 

 of negatively charged rays. These rays are formed by a normal 

 positively charged ray picking up two electrons. On the negative 

 spectrum of chlorine only two lines, 85 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 complex 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 I., II., III. and IV. show the results with chlorine 

 taken with different magnetic field strengths. 



The objection has been raised on many occasions that if chlorine 

 consists of isotopes, how is it that its atomic weight has been deter- 

 mined so accurately and so consistently by different chemists ? The 

 obvious explanation of this appears to be that all the accurate deter- 

 minations 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 deter- 

 mined, exactly as in the case of lead. 



The mass spectrum of argon shows an exceedingly bright line 

 at 40, with second order line at 29, and third order line at 13 1/3. 

 The last is particularly well placed between known reference lines, 

 and its measurement showed that the triply- charged atom causing 

 it had a mass 40 very exactly. Now the accepted atomic weight 

 of argon is less than 40, so the presence of a lighter isotope was 

 suggested. This was found at 36, and has now been fully sub- 

 stantiated ; its presence to the extent of about 3 per cent is sufficient 

 to account for the mean atomic weight obtained by density deter- 

 minations. 



The elements hydrogen and helium presented peculiar difficulties, 

 as their lines were too far removed from the reference lines for direct 

 comparison. By means of a special "bracketing" method moderately 

 accurate values were obtained. Helium appears to be exactly 4 on 

 the oxygen scale, but hydrogen is definitely greater than unity. The 

 value obtained agrees very well with that already arrived at by 

 chemical methods, namely, 1 ' 008. At the same time, measurements 

 of the 3 line, first observed by Sir J. J. Thomson, were made which 

 came out at 3*024, satisfactorily proving it to be due to triatomic 

 hydrogen. 



Krypton and xenon gave surprisingly complex results, the former 



