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220 SCIENCE PROGRESS 



the mass of He is less than twice that of H2. On carrying on 

 the ratio in the same way to the second order Hne of oxygen, 

 it is found that heHum is exactly 4, but that hydrogen gives 

 a value the same as that obtained by chemists, namely i-oo8. 



Krypton {At. Wt. 82-92) and Xenon {At. Wt. 130-2). — The 

 results with these elements were particularly interesting, as 

 they proved to have a quite unexpected complexity. Also, 

 as they are inactive gases, there is no question of confusion of 

 the evidence due to possible hydrogen compounds. Krypton 

 gives 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 intensities 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 (28) 

 with the highest accuracy. The singly and doubly charged 

 krypton clusters can be seen to the right and left of Spec- 

 trum VIII. 



Only the minutest trace of Xenon was available to deter- 

 mine its constitution. The five lines to the right of Spec- 

 trum IX indicate that provisionally it may be said to be a 

 mixture of five isotopes of masses 128, 130, 131, 133, 135. 



Mercury {At. Wt. 200-6). — ^Owing to the presence of mer- || 

 cury vapour (which is generally beneficial to the smooth M 

 running of the discharge), the lines of this element appear on fl 

 nearly all the plates taken. They do so as a series of blurred 

 clusters of decreasing intensity around points corresponding 

 to 200, 100, 66-6, 50, etc., some of which are indicated in 

 the spectra reproduced. It may be stated provisionally that 

 they indicate a strong component 202, a weak one 204, and a 

 strong band from 197 to 200 containing three or four more, M 

 unresolvable at present. ^ 



It is not proposed to discuss in detail all the other elements 

 analysed, particularly as the evidence in many cases is by no 

 means simple ; a table of the results so far obtained is given 

 at the end of the article. The case of the element bromine 

 is of some special interest, for, in spite of the fact that its 

 chemical combining weight is almost exactly a whole number 

 (80), it actually consists of approximately equal parts of 79 

 and 81. Boron is the lightest element found to consist of 

 isotopes. 



The Whole-Number Rule. — 'The most important generali- 

 sation yielded by these experiments is the remarkable fact 

 that all masses — atomic or molecular, element or compound — 

 so far measured are whole numbers on the oxygen scale within 

 about 0-05 or less. It is perhaps premature to state that this 

 relation is true of all elements, but the number and variety of 



