PROGRESS IN PHYSICS—THOMSON. 187 
ume of half a millionth of a cubic centimeter. When stated in this 
form the quantity seems exceedingly small, but in this small volume 
there are about ten million million molecules. Now the population 
of the earth is estimated 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 centimeter. Rutherford has 
shown that we can detect the presence of a single a-particle. Now 
the particle is a charged atom of helium; if this atom had been un- 
charged 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 ob- 
tain a knowledge of the ultimate structure of electricity before we 
arrive at a corresponding degree of certainty with regard to the 
structure of matter. 
We have already made considerable progress in the task of dis- 
covering 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 ex- 
ceedingly small compared with even the smallest atom, for the mass 
of the unit is only one seventeen-hundredth part of the mass of an 
atom of hydrogen; that its radius is only 10° centimeter, 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 
hight. 
The discovery of these corpuscles is an interesting example of the 
way nature responds to the demands made upon her by mathema- 
ticlans. 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 hopelessly 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 
