So ae 
PRESIDENTIAL ADDRESS. 381. 
direct experiment. 'This discussion would not be complete without a refer- 
ence to an important determination of e from theoretical considerations by 
Planck. From the theory of the distribution of energy in the spectrum of a 
hot body, Planck found that e=4:69x10-", and N=2:80x10"°. For 
reasons that we cannot enter into here, this theoretical deduction must be 
given great weight. 
When we consider the great diversity of the theories and methods which 
have been utilised to determine the values of the atomic constants e and N, 
and the probable experimental errors, the agreement among the numbers 
is remarkably close. This is especially the case in considering the more 
recent measurements by very different methods, which are far more reliable 
than the older estimates. It is difficult to fix on one determination as more 
deserving of confidence than another ; but I may be pardoned if I place some 
reliance on the radioactive method previously discussed, which depends on 
the charge carried by the a particle. The value obtained in this way is 
not only in close agreement with the theoretical estimate of Planck, but is 
in dair agreement with the recent determinations by several other distinct 
methods. We may consequently conclude that the number of molecules in 
a cubic centimetre of any gas at standard pressure and temperature is 
about 2°77X10"°, and that the value of the fundamental unit of quantity 
of electricity is about 4°65 X10—’° electrostatic units. From these data it is 
a simple matter to deduce the mass of any atom whose atomic weight is 
known, and to determine the values of a number of related atomic and mole- 
cular magnitudes. 
There is now no reason to view the values of these fundamental constants 
with scepticism, but they may be employed with confidence in calculations 
to advance still further our knowledge of the constitution of atoms and 
molecules. There will no doubt be a great number of investigations in the 
future to fix the values of these important constants with the greatest 
possible precision; but there is every reason to believe that the values are 
already known with reasonable certainty, and with a degree of accuracy far 
greater than it was possible to attain a few years ago. The remarkable 
agreement in the values of e and N, based on so many different theories, of 
itself affords exceedingly strong evidence of the correctness of the atomic 
theory of matter, and of electricity, for it is difficult to believe that such 
concordance would show itself if the atoms and their charges had no real 
existence. 
There has been a tendency in some quarters to suppose that the develop- 
ment of physics in recent years has cast doubt on the validity of the atomic 
theory of matter. This view is quite erroneous, for it will be clear from 
the evidence already discussed that the recent discoveries have not only 
greatly strengthened the evidence in support of the theory, but have given 
an almost direct and convincing proof of its correctness. The chemical atom 
as a definite unit in the subdivision of matter is now fixed in an im- 
pregnable position in science. Leaving out of account considerations of 
etymology, the atom in chemistry has long been considered to refer only 
to the smallest unit of matter that enters into ordinary chemical combina- 
tion. There is no assumption made that the atom itself is indestructible 
and eternal, or that methods may not ultimately be found for its sub- 
division into still more elementary units. The advent of the electron has 
shown that the atom is not the unit of smallest mass of which we have 
cognisance, while the study of radioactive bodies has shown that the 
atoms of a few elements of high atomic weight are not permanently stable, 
but break up spontaneously with the appearance of new types of matter. 
These advances in knowledge do not in any way invalidate the position of 
the chemical atom, but rather indicate its great importance as a sub- 
division of matter whose properties should be exhaustively studied. 
The proof of the existence of corpuscles or electrons with an apparent 
