882 TRANSACTIONS OF SECTION A. 
mass very small compared with that of the hydrogen atom, marks an im- 
portant stage in the extension of our ideas of atomic constitution. This 
discovery, which has exercised a profound influence on the development of 
modern physics, we owe mainly to the genius of the President of this Associa- 
tion. The existence of the electron as a distinct entity is established by 
similar methods and with almost the same certainty as the existence of 
individuala particles. While it has not yet been found possible to detect a 
single electron by its electrical or optical effect, and thus to count the 
number directly as in the case of the a particles, there seems to be no reason 
why this should not be accomplished by the electric method. The effect to be 
anticipated for a single B particle is much smaller than that due to an 
a particle, but not too small for measurement. In this connection it is of 
interest to note that Regener has observed evidence of scintillations produced 
by the 8 particles of radium falling on a screen of platinocyanide of barium, 
but the scintillations are too feeble to count with certainty. 
Experiment has shown that the apparent mass of the electron varies 
with its speed, and, by comparison of theory with experiment, it has been 
concluded that the mass of the electron is entirely electrical in origin and 
that there is no necessity to assume a material nucleus on which the 
electrical charge is distributed. While there can be no doubt that electrons 
can be released from the atom or molecule by a variety of agencies and, 
when in rapid motion, can retain an independent existence, there is still 
much room for discussion as to the actual constitution of electrons, if such 
a term may be employed,’ and of the part they play in atomic structure. 
There can be little doubt that the’ atom is a complex system, consisting of 
a number of positively and negatively charged masses which are held in 
equilibrium mainly by electrical forces; but it is difficult to assign the 
relative importance of the réle played by the carriers of positive and 
negative electricity. While negative electricity can exist as a separate 
entity in the electron, there is yet no decisive proof of the existence of a 
corresponding positive electron. It is not known how much of the mass of 
an atom is due to electrons or other moving charges, or whether a type of 
mass quite distinct from electrical mass exists. Advance in this direction 
must be delayed until a clearer knowledge is gained of the character and 
structure of positive electricity and of its relation to the negative electron. 
The general experimental evidence indicates that electrons play two 
distinct réles in the structure of the atom, one as lightly attached and 
easily removable satellites or outliers of the atomic system, and the other 
as integral constituents of the interior structure of the atom. The former, 
which can be easily detached or set in vibration, probably play an im- 
portant part in the combination of atoms to form molecules, and in the 
spectra of the elements; the latter, which are held in place by much 
stronger forces, can only be released as a result of an atomic explosion 
involving the disintegration of the atom. For example, the release of an 
electron with slow velocity by ordinary laboratory agencies does not appear 
to endanger the stability of the atom, but the expulsion of a high speed 
electron from a radioactive substance accompanies the transformation of 
the atom. 
The idea that the atoms of the elements may be complex structures, made 
up either of lighter atoms or of the atoms of some fundamental substance, 
has long been familiar to science. So far no direct evidence has been 
obtained of the possibility of building up an atom of higher atomic weight 
from one of lower atomic weight, but in the case of the radioactive 
substances we have decisive and definite evidence that certain elements 
show the converse process of disintegration. It may be significant that 
this process has only been observed in the atoms of highest atomic weights, 
like those of uranium, thorium, and radium. With the exception possibly 
