238 



ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



In the same way He was compared with ++ , and H 3 , obtained from 

 KOH by Sir J. J. Thomson's bombardment method, with C ++ . 



The method has some definite advantages and some disadvantages. 

 It is not proposed to discuss these in detail at present. The values 

 obtained by its use can be checked in the ordinary way by comparing 

 He with C ++ and H 3 with He, these pairs being close enough together 

 for the purpose. The following table gives the range of values ob- 

 tained from the most reliable plates : 



From these figures it is safe to conclude that hydrogen is a " pure " 

 element and that its atomic weight, determined with such consistency 

 and accuracy by chemical methods (1.008), is the true mass of its 

 atom. 



The above results incidentally appear to settle the nature of the 

 molecule H 3 beyond doubt. 



KRYPTON (AT. WT. 82.92) AND XENON (AT. WT. 130.2). 



The results with these elements were particularly interesting. The 

 only source available, for which the author is indebted to Sir J. J.. 

 Thomson, was the remains of two small samples of gas from evapo- 

 rated liquid air kindly supplied by Sir James Dewar some years ago 

 for examination by the " parabola " method. Both samples contained 

 nitrogen, oxygen, argon, and krypton, but xenon was only detected 

 in one and its percentage in that must have been quite minute. 

 Krypton is characterized by a remarkable group of five strong lines 

 at 80, 82, 83, 84, 86, and a faint sixth at 78. This group or cluster 

 of isotopes is beautifully reproduced with the same relative values 

 of intensity in the second, and fainter still in the third order. These 

 multiply-charged clusters give most reliable values of mass, as the 

 second order can be compared with A (40) and the third with CO 

 or N 2 (28) with the highest accuracy. It will be noted that one 

 member of each group is obliterated by the reference line, but not 

 the same one. The singly and doubly charged krypton clusters can 

 be seen to the right and left of Spectrum VIII. It will be noticed 

 that krypton is the first element examined which shows unmistakable 

 isotopes differing by one unit only. 



On the krypton plates taken with the greatest magnetic field faint 

 but unmistakable indications of lines in the region of 130 could just 



