PRESIDENTIAL ADDRESS. old 
of radium. But we can go further and confirm the result by counting the 
number of a particles by an entirely distinct method. Sir William Crookes 
has shown that when the a rays are allowed to fall upon a screen of phos- 
phorescent zinc sulphide, a number of brilliant scintillations are observed. 
It appears as if the impact of each a particle produced a visible flash of 
light where it struck the screen. Using suitable screens the number of 
scintillations per second on a given area can be counted by means of a 
microscope. It has been shown that the number of scintillations deter- 
mined in this way is equal to the number of impinging a particles when 
counted by the electric method. This shows that the impact of each a par- 
ticle on the zinc sulphide produces a visible scintillation. There are thus 
two distinct methods—one electrical, the other optical—for detecting the 
emission of a single a particle from radium. The next question to con- 
sider is the nature of the a particle itself. The general evidence indicates 
that the a particle is a charged atom of helium, and this conclusion was 
decisively verified by Rutherford and Royds by showing that helium 
appeared in an exhausted space into which the a particles were fired. 
The helium, which is produced by radium, is due to the accumulated 
a particles which are so continuously expelled from it. If the rate of 
production of helium from radium is measured, we thus have a means of 
determining directly how many a particles are required to form a given 
volume of helium gas. This rate of production has recently been measured 
accurately by Sir James Dewar. He has informed me that his final 
measurements show that one gram of radium in radioactive equilibrium 
produces 0°46 cubic millimetres of helium per.day, or 5°32X10 ~® cubic 
millimetres per second. Now from the direct counting experiments it is 
known that 13°6x10” a particles are shot out per second from one gram 
of radium in equilibrium. Consequently it requires 2°56X10" a particles 
to form one cubic centimetre of helium gas at standard pressure and 
temperature. 
From other lines of evidence it is known that all the a particles from 
whatever source are identical in mass and constitution. It is not then 
unreasonable to suppose that the a particle, which exists as a separate 
entity in its flight, can exist also as a separate entity when the a particles 
are collected together to form a measurable volume of helium gas, or, in 
other words, that the a particle on losing its charge becomes the fundamental 
unit or atom of helium. In the case of a monatomic gas like helium, where 
the atom and molecule are believed to be identical, no difficulty of deduction 
arises from the possible combination of two or more atoms to form a complex 
molecule. 
We consequently conclude from these experiments that one cubic centi- 
metre of helium at standard pressure and temperature contains 2°56X10" 
atoms. Knowing the density of helium, it at once follows that each atom 
of helium has a mass of 6°8X10-*4 grams, and that the average distance 
apart of the molecules in the gaseous state at standard pressure and 
temperatures is 3°4X10 7 centimetres. 
The above result can be confirmed in a different way. It is known that 
the value of e/m for the a particle is 5,070 electromagnetic units. The 
positive charge carried by each a particle has been deduced by measuring 
the total charge carried by a counted number of a particles. Its value is 
9°3X10-” electrostatic units, or 3°1X10—% electromagnetic units. Sub- 
stituting this number in the value of e/m, it is seen that m, the mass of the 
a particle, is equal to 6°1X10-*4 grams—a value in fair agreement with the 
number previously given. 
I trust that my judgment is not prejudiced by the fact that I have taken 
some share in these investigations; but the experiments, taken as a whole, 
appear to me to give an almost direct and convincing proof of the atomic 
hypothesis of matter. By direct counting, the number of identical entities 
