SOME ASPECTS OF THE ATOMIC THEORY 579 



sufficiently sensitive to detect them, or, to put it another way, 

 unless we had a better test for a man than for an unelectrified 

 molecule, we should be unable to find out that the earth was 

 inhabited. ... A billion unelectrified atoms may escape our 

 observation, whereas a dozen or so electrified ones are detected 

 without difficulty." Owing to the charge the ions may be sorted 

 out by suitable application of electric and magnetic forces 

 (positive ray analysis), and we get from a mixed gas a limited 

 number of sharply defined streams. " This shows that all the 

 atoms of an element are alike ; this has sometimes been 

 questioned." The possibility that the weights of the several 

 atoms of the same elements may differ by varying small amounts 

 on either side of a mean value, much as the individual velocities 

 of gas molecules differ from the mean velocity according to the 

 kinetic theory, is, of course, old. As we read in another con- 

 nection, " The statistician is content to know that the average 

 height of male adults is, say 5 ft. 6 in. and their waist measure- 

 ment 3 ft., but it is evident such knowledge would be a very 

 unsatisfactory equipment for one's tailor." It is interesting to 

 know that the results of the positive ray method of gas analysis 

 give no support to such a view, especially as the older line of 

 argument, derived from the extreme sharpness of the individual 

 lines in the spectrum of an element, must now be reconsidered 

 and probably rejected altogether. Spectroscopic quantities seem 

 to depend on atomic charge and not on atomic mass. But that 

 a totally opposite conclusion in the single case of the element 

 neon has been drawn from the work of Ashton by this method 

 in the Cavendish Laboratory is not referred to, nor is the newer 

 theory of isotopic elements founded on the study of radioactive 

 change. Such elements, so called because they occupy the 

 " same place " in the periodic table, are identical in chemical 

 properties and in such physical properties as do not depend 

 directly on the mass, but they may differ by whole units in 

 atomic weight. The work on radioactive change, which has 

 resulted in much that is really new and important in the Atomic 

 Theory and in knowledge of the structure of the atom, it is scarcely 

 necessary to refer to here in detail, as much of it is contained 

 in the recent discussion at the Royal Society on Atomic Structure 

 (see Science Progress, July 1914, p. 169). As, however, the 

 knowledge of the existence of isotopes brings a new possibility to 

 be taken into account in the discussion of the cause of the varia- 



