PRESIDENTIAL ADDRESS. 3883 
of potassium, there is no reliable evidence that a similar process takes 
place in other elements. ‘The transformation of the atom of a radioactive 
substance appears to result from an atomic explosion of great intensity 
in which a part of the atom is expelled with great speed. 1n the majority 
of cases, an a particle or atom of helium is ejected, in some cases a high- 
speed electron, while a few substances are transformed without the 
appearance of a detectable radiation. The fact that the a particles from 
a simple substance are all ejected with an identical and very high velocity 
suggests the probability that the charged helium atom before its expulsion 
is in rapid orbital movement in the atom. There is at present no detinite 
evidence of the causes operative in these atomic transformations. 
Since in a large number of cases the transformations of the atoms are 
accompanied by the expulsion of one or more charged atoms of helium, it is 
difficult to avoid the conclusion that the atoms otf the radioactive elements 
are built up, in part at least, of helium atoms. It is certainly very 
remarkable and may prove of great significance, that helium, which is 
regarded from the ordinary chemical standpoint as an inert element, plays 
such an important part in the constitution of the atoms of uranium, 
thorium, and radium. 
The study of radioactivity has not only thrown great light on the 
character of atomic transformations, but it has also led to the development 
of methods for detecting the presence of almost infinitesimal quantities of 
radioactive matter. It has already been pointed out that two methods—one 
electrical, the other optical—have been devised for the detection of a single 
a particle. By the use of the optical or scintillation method, it is possible 
to count with accuracy the number of a particles when only one is expelled 
per minute. It is not a difficult matter, consequently, to tollow the trans- 
formation of any radioactive substance in which only one atom breaks up 
per minute, provided that an a particle accompanies the transformation. 
in the case of a rapidly changing substance like the actinium emanation, 
which has a half period of 3-7 seconds, it is possible to detect with certainty 
the presence, if not of a single atom, at any rate of a few atoms, while 
the presence of a hundred atoms would in some cases give an inconveniently 
large effect. ‘The counting of the scintillations affords an exceedingly power- 
ful and direct quantitative method of studying the properties of radioactive 
substances which expel a particles. Not only is it a simple matter to count 
the number of a particles which are expelled in any given interval, but it 
is possible, for example, by suitably arranged experiments to decide whether 
one, two or more a particles are expelled at the disintegration of a single atom. 
The possibility of detection of a single atom of matter has opened up 
a new field of investigation in the study of discontinuous phenomena. 
For example, the experimental law of transformation of radioactive matter 
expresses only the average rate of transformation, but by the aid of the 
scintillation or electric method it is possible to determine directly by 
experiment the actual interval between the disintegration of successive 
atoms and the probability law of distribution of the a particles about the 
average value. 
Quite apart from the importance of studying radioactive changes, the 
radiations from active bodies provide very valuable information as to 
the effects produced by high velocity particles in traversing matter. The 
three types of radiation, the a, 8, and y rays, emitted from active bodies, 
differ widely in character and their power of penetration of matter. The 
a particles, for example, are completely stopped by a sheet of notepaper, 
while the y rays from radium can be easily detected after traversing twenty 
centimetres of lead. The differences in the character of the absorption of 
the radiations are no doubt partly due to the difference in type of ttle 
radiation and partly due to the differences of velocity, 
