142 Dr. F. W. Aston on the 



these may possibly have been due to isotopes of tin. This 

 supposition is slightly strengthened by the appearance of a 

 still fainter group of odd integral mass containing the lines 

 155, 157, etc., which might be isotopic tin monochlorides. It 

 has not been found possible to repeat this result, so that no 

 reliance is to be put upon it. 



Further experiments with Xenon. 



The first results obtained from this gas (Phil. Mag. xxxix. 

 p. 623) were unsatisfactory as the material available only 

 contained a small percentage of it. Owing to the kindness of 

 Prof. Travers and Dr. Masson, who have recently isolated 

 some quantity of the heavier inactive constituents of the air, 

 a sample of gas was supplied which contained a large pro- 

 portion of xenon though it was by no means free from 

 krypton. This yielded some excellent mass spectra, which 

 not only served to correct the figures given for the five 

 isotopes discovered previously, but also indicated the possi- 

 bility of two additional ones. 



The absolute position of the group on the mass scale was 

 satisfactorily fixed by means of the second order line of the 

 strongest member, which fortunately lies outside the third 

 order mercury group. This gave constant and accurate 

 values corresponding to 64*5 (Spectrum III.). The five 

 strong lines of xenon are therefore 129, 131, 132, 134, 136. 

 On the left of the first there was to be seen on many of the 

 plates distinct indications of a faint component 128. Also 

 the darkening between the lines 129 and 131 appears 

 decidedly greater than that to be expected from ordinary 

 halation and suggests the possibility of a seventh isotope 130. 



Lines due to negatively charged Chlorine. 



The mass spectrograph is designed to give the minimum 

 chance of collision after leaving the first cathode slit, so that 

 under normal working conditions negative lines, which can 

 only result from a positively charged particle capturing two 

 electrons before reaching the analysing fields, are practically 

 non-existent. If, however, the liquid air which cools the 

 charcoal exhausting the space between the slits is omitted the 

 pressure rises and, at the expense of intensity, negatively 

 charged rays are formed. If now both the electric and 

 magnetic fields are reversed in sign, the mass spectra of the 

 negatively charged particles is obtained. 



Such negative mass spectra are naturally much less intense 



