12 PRESIDENT’S ADDRESS. 
a corresponding degree of certainty with regard to the structure of 
matter. 
We have already made considerable progress in the task of discover- 
ing what the structure of electricity is. We have known for some 
time that of one kind of electricity—the negative—and a very interesting 
one it is. We know that negative electricity is made up of units all 
of which are of the same kind; that these units are exceedingly small 
compared with even the smallest atom, for the mass of the unit is only 
17qso Part of the mass of an atom of hydrogen; that its radius is only 
10-* centimetre, and that these units, ‘ corpuscles’ as they have 
been called, can be obtained from all substances. The size of these 
corpuscles is on an altogether different scale from that of atoms; the 
volume of a corpuscle bears to that of the atom about the same relation 
as that of a speck of dust to the volume of this room. Under suitable 
conditions they move at enormous speeds which approach in some 
instances the velocity of light. 
The discovery of these corpuscles is an interesting example of the 
way Nature responds to the demands made upon her by mathematicians. 
Some years before the discovery of corpuscles it had been shown by 
a mathematical investigation that the mass of a body must be increased 
by a charge of electricity. This increase, however, is greater for small 
bodies than for large ones, and even bodies as small as atoms are hope- 
lessly too large to show any appreciable effect; thus the result seemed 
entirely academic. After a time corpuscles were discovered, and these 
are so much smaller than the atom that the increase in mass due to the 
charge becomes not merely appreciable, but so great that, as the 
experiments of Kaufmann and Bucherer have shown, the whole of the 
mass of the corpuscle arises from its charge. 
We know a great deal about negative electricity ; what do we know 
about positive electricity? Is positive electricity molecular in structure ? 
Is it made up into units, each unit carrying a charge equal in magnitude 
though opposite in sign to that carried by a corpuscle? Does, or does not, 
this unit differ, in size and physical properties, very widely from the 
corpuscle? We know that by suitable processes we can get corpuscles ” 
out of any kind of matter, and that the corpuscles will be the same from 
whatever source they may be derived. Is a similar thing true for positive 
electricity? Can we get, for example, a positive unit from oxygen of 
the same kind as that we get from hydrogen? 
For my own part, I think the evidence is in favour of the view that 
we can, although the nature of the unit of positive electricity makes the 
proof much more difficult than for the negative unit. 
Tn the first place we find that the positive particles—‘ canalstrahlen ’ 
is their technical name—discovered by our distinguished guest, Dr. Gold- 
stein, which are found when an electric discharge passes through a highly 
rarefied gas, are, when the pressure is very low, the same, whatever may 
