PRESIDENT’S ADDRESS. 11 
We know from these investigations that electricity, like matter, is 
molecular in structure, that just as a quantity of hydrogen is a collection 
of an immense number of small particles called molecules, so a charge of 
electricity is made up of a great number of small charges, each of a 
perfectly definite and known amount. 
Helmholtz said in 1880 that in his opinion the evidence in favour of 
the molecular constitution of electricity was even stronger than that in 
favour of the molecular constitution of matter. How much stronger is 
that evidence now, when we have measured the charge on the unit and 
found it to be the same from whatever source the electricity is obtained. 
Nay, further, the molecular theory of matter is indebted to the molecular 
theory of electricity for the most accurate determination of its funda- 
mental quantity, the number of molecules in any given quantity of an 
elementary substance. 
The great advantage of the electrical methods for the study of the 
properties of matter is due to the fact that whenever a particle is electrified 
it is very easily identified, whereas an uncharged molecule is most elusive ; 
and it is only when these are present in immense numbers that we are 
able to detect them. A very simple calculation will illustrate the dif- 
ference in our power of detecting electrified and unelectrified molecules. 
The smallest quantity of unelectrified matter ever detected is probably 
that of neon, one of the inert gases of the atmosphere. Professor Strutt 
has shown that the amount of neon in 4, of a cubic centimetre of the air 
at ordinary pressures can be detected by the spectroscope; Sir William 
Ramsay estimates that the neon in the air only amounts to one part of 
neon in 100,000 parts of air, so that the neon in s4, of a cubic centimetre 
of air would only occupy at atmospheric pressure a volume of half a 
millionth of a cubic centimetre. When stated in this form the quan- 
tity seems exceedingly small, but in this small volume there are about 
ten million million molecules. Now the population of the earth is esti- 
mated at about fifteen hundred millions, so that the smallest number of 
molecules of neon we can identify is about 7,000 times the population of 
the earth. In other words, if we had no better test for the existence of a 
man than we have for that of an unelectrified molecule we should come 
to the conclusion that the earth is uninhabited. Contrast this with 
our power of detecting electrified molecules. We can by the electrical 
method, even better by the cloud method of C. T. R. Wilson, detect the 
presence of three or four charged particles in a cubic centimetre. 
Rutherford has shown that we can detect the presence of a single 
a particle. Now the a particle is a charged atom of helium; if this atom 
had been uncharged we should have required more than a million million 
of them, instead of one, before we should have been able to detect them. 
We may I think conclude, since electrified particles can be studied 
with so much greater ease than unelectrified ones, that we shall obtain 
a knowledge of the ultimate structure of electricity before we arrive at 
