A.—_MATHEMATICAL AND PHYSICAL SCIENCES 25 
delicate methods of radio-activity. These ‘ radio-active indicators’ 
have been applied to problems of chemistry, otherwise unapproachable, 
such as the rates of molecular diffusion in the liquid state and the move- 
ments of compounds of heavy elements in the sap of living organisms. 
The application of the theory of isotopes to elements generally was 
another matter. The idea that ordinary elements could consist of atoms 
of different mass received great opposition, for it appeared quite incom- 
patible with such facts as the constancy of chemical atomic weight, the 
apparently perfect homogeneity of elementary gases, and the almost 
incredible invariability of such accurately measurable constants as the 
electrical conductivity of mercury independent of its source. This 
reluctance of orthodox science to accept the theory was, I think, a perfectly 
natural and healthy reaction. Criticism very seldom destroys enthusiasm 
and is usually the best stimulant to further research, whereas too immediate 
a welcome of a new and sensational idea, the outstanding fault of the lay 
press in dealing with science, may lead to waste of effort. It appears to 
me a very regrettable thing that, of recent years, it has been repeatedly 
necessary for experienced research workers to waste their time on the 
thankless task of disproving the claims of well-meaning victims of self- 
deception, of whom Blondlot, with his N-rays, is the classical example. 
The only satisfactory criterion, a method of comparing the masses of 
individual atoms, was at the time in process of development. This was 
Sir J. J. Thomson’s ‘ parabola’ method of positive ray analysis, and here 
at first all the results seemed to support Dalton’s postulate, indeed the 
appearance on a sensitive screen of a clear-cut parabolic streak, caused 
by the impact of the atoms of hydrogen, was the first experimental proof 
that it was in any sense true of any element, previously it had been purely 
an article of scientific faith. Hydrogen, carbon, nitrogen, and oxygen, 
present either as atoms or molecules, gave parabolas in the positions 
expected, and it was only when the rare gas neon was examined that an 
anomaly was observed. Neon, however pure, always gave two parabolas, 
a strong one at 20 and a weak one at 22. Referring to the latter in 
January 1913, Sir J. J. Thomson said: ‘ The origin of this line presents 
many points of interest ; there are no known gaseous compounds of any 
of the recognised elements which have this molecular weight. Again, 
if we accept Mendeléef’s Periodic Law, there is no room for a new element 
with this atomic weight. . . . There is, however, the possibility that we 
may be interpreting Mendeléef’s law too rigidly, and that in the neigh- 
bourhood of the atomic weight of neon there may be a group of two or 
more elements with similar properties, just as in another part of the table 
we have the group iron, nickel and cobalt.’ 
It was my privilege to be associated with him in this work, and as his 
attention was fully occupied with the investigation of a parabola of mass 3 
—now known to be triatomic hydrogen—it fell to my lot to search for 
a proof that neon was not homogenious. This I endeavoured to do by 
partial separation of its hypothetical constituents, using as a test its density 
measured by a quartz micro-balance specially designed for the purpose. 
The first method, that of fractional distillation from charcoal cooled with 
liquid air, failed, as we now know was inevitable. ‘The second, diffusion 
