24 SECTIONAL ADDRESSES 
radio-activity. In the rapid development of this, with which the school 
of Rutherford is so closely associated, the effects of individual atoms, as 
opposed to those of vast multitudes, were observed for the first time. 
Chemists could examine elements in the actual process of the making. 
In 1906 Boltwood observed that his newly discovered element ionium 
was so similar to thorium that if, by chance, their salts became mixed 
it was impossible to separate them by any chemical process. Other 
chemical identities among the products of radio-activity were soon ob- 
served and the most painstaking and delicate methods failed to effect or 
detect the slightest separation. 
Discussing these, Soddy, in 1910, boldly stated: ‘'These regularities 
may prove to be the beginning of some embracing generalisation, which 
will throw light, not only on radio-active processes, but on elements in 
general and the Periodic Law. . . . Chemical homogeneity is no longer 
a guarantee that any supposed element is not amixture of several of different 
atomic weights, or that any atomic weight is not merely a mean number.’ 
The generalisation underlying his views was the law connecting radio- 
activity and chemical change, in the discovery and enunciation of which 
he played so prominent a part. This law asserts that a radio-active 
element when it loses an alpha particle goes back two places in the periodic 
table ; when it loses a beta particle it goes forward one place. It follows 
that by the loss of one alpha particle followed by two beta particles, the 
atom, though weighing four units less, will have regained its nuclear 
charge and returned to its original place. 
Such changes result in bodies to which Soddy applied the following 
words: ‘ The same algebraic sum of the positive and negative charges 
in the nucleus when the arithmetical sum is different gives what I call 
‘* isotopes ’”! or “‘ isotopic elements ”’ because they occupy the same place 
in the periodic table. ‘They are chemically identical, and save only as 
regards the relatively few physical properties which depend upon atomic 
mass directly, physically identical also.’ It was fortunately possible to 
put these revolutionary views to an experimental test in the case of one 
element—lead, the final inactive product of the thorium and uranium 
transformations. Uranium of atomic weight 238 loses eight alpha 
particles to become lead of atomic weight 206, while thorium of mass 232 
loses six to become lead of atomic weight 208. Soddy maintained that the 
lead found in uranium minerals should be lighter, and that in thorium 
minerals heavier than ordinary lead of atomic weight 207-2. 
The complete chemical inseparability of the heavy isotopes formed in 
radio-active processes passed the most stringent tests and was soon 
accepted. It was later put to a most ingenious and elegant use by Paneth 
and Hevesy, who, by adding to an inactive element a small quantity of 
its radio-active isotope, gave it, so to speak, an indelible label by which 
its movements and reactions could be followed by the almost infinitely 
1 Of recent years the word ‘ isotope’ has changed its meaning, and is now used, 
for lack of another, to designate any atomic species. In the same way the mean- 
ing of the word ‘ mass-spectograph ’ applied by me to one special type of instru- 
ment has now been extended to any form capable of analysing mass-rays. Such 
changes, though troublesome, are inevitable for the language of science is a 
living rather than a dead one.—F. W. A. 
