PROGRESS IN PHYSICS—-THOMSON. 201 
Mr. Campbell has recently found that potassium, though far in- 
ferior in this respect to any of the substances I have named, emits an 
appreciable amount of radiation, the amount depending only on the 
quantity of potassium, and being the same whatever the source from 
which the potassium is obtained or whatever the elements with which 
it may be in combination. 
The radiation emitted by these substances is of three types known 
as a-, B-, and y-rays. The a-rays have been shown by Rutherford to 
be positively electrified atoms of helium, moving with speeds which 
reach up to about one-tenth of the velocity of hght. The B-rays are 
negatively electrified corpuscles, moving 1n some cases with very 
nearly the velocity of light itself, while the y-rays are unelectrified, 
and are analogous to the Rontgen rays. 
The radioactivity of uranium was shown by Crookes to arise from 
something mixed with the uranium, and which differed sufficiently in 
properties from the uranium itself to enable it to be separated by 
chemical analysis. He took some uranium, and by chemical treatment 
separated it into two portions, one of which was radioactive and the 
other not. 
Next, Becquerel found that if these two portions were kept for 
several months, the part which was not radioactive to begin with re- 
gained radioactivity, while the part which was radioactive to begin 
with had lost its radioactivity. These effects and many others receive 
a complete explanation by the theory of radioactive change which we 
owe to Rutherford and Soddy. 
According to this theory, the radioactive elements are not perma- 
nent, but are gradually breaking up into elements of lower atomic 
weight; uranium, for example, is slowly breaking up, one of the 
products being radium, while radium breaks up into a radioactive 
gas called radium emanation, the emanation into another radioactive 
substance, and so on, and that the radiations are a kind of swan’s 
song emitted by the atoms when they pass from one form to another ; 
that, for example, it is when a radium atom breaks up and an atom 
of the emanation appears that the rays which constitute the radio- 
activity are produced. 
Thus, on this view the atoms of the radioactive elements are not 
immortal; they perish after a life whose average value ranges from 
thousands of millions of years in the case of uranium to a second or 
so in the case of the gaseous emanation from actinium. 
When the atoms pass from one state to another they give out large 
stores of energy, thus their descendants do not inherit the whole of 
their wealth of storedup energy; the estate becomes less and less 
wealthy with each generation; we find, in fact, that the politician, 
when he imposes death duties, is but imitating a process which has 
been going on for ages in the case of these radioactive substances. 
