PROGRESS IN PHYSICS—THOMSON. 189 
through the hydrogen molecules than the positive particles derived 
from hydrogen itself, which would, on this view, be of the size and 
weight of the light hydrogen molecules. Wellisch found that the 
velocities of both the positive and negatives particles through the mix- 
ture were the same as the velocities through pure hydrogen, although 
in the one case the ions had originated from methyliodide and in the 
other from hydrogen; a similar result was obtained when carbon 
tetrachloride, or mercury methyl, was used instead of methyliodide. 
These and similar results lead to the conclusion that the atom of 
the different chemical elements contain definite units of positive as 
well as of negative electricity, and that the positive electricity, like 
the negative, is molecular in structure. 
The investigations made on the unit of positive electricity show 
that it is of quite a different kind from the unit of negative, the mass 
of the negative unit is exceedingly small compared with any atom, 
the only positive units that up to the present have been detected are 
quite comparable in mass with the mass of an atom of hydrogen; in 
fact, they seem equal to it. This makes it more difficult to be certain 
that the unit of positive electricity has been isolated, for we have to 
be on our guard against its being a much smaller body attached to 
the hydrogen atoms which happen to be present in the vessel. If the 
positive units have a much greater mass than the negative ones, they 
ought not to be so easily deflected by magnetic forces when moving 
at equal speeds; and, in general, the insensibility of the positive par- 
ticles to the influence of a magnet is very marked; though there are 
cases when the positive particles are much more readily deflected, 
and these have been interpreted as proving the existence of positive 
units comparable in mass with the negative ones. I have found, how- 
ever, that in these cases the positive particles are moving very slowly, 
and that the ease with which they are deflected is due to the smallness 
of the velocity and not to that of the mass. It should, however, be 
noted that M. Jean Becquerel has observed in the absorption spectra 
of some minerals and Professor Wood in the rotation of the plane 
of polarization by sodium vapor, effects which could be explained 
by the presence in the substances of positive units comparable in 
mass with corpuscles. This, however, is not the only explanation 
which can be given of these effects, and at present the smallest posi- 
tive electrified particles of which we have direct experimental evi- 
dence have masses comparable with that of an atom of hydrogen, 
A knowledge of the mass and size of the two units of electricity, 
the positive and the negative, would give us the material for con- 
structing what may be called a molecular theory of electricity, and 
would be a starting point for a theory of the structure of matter; 
for the most natural view to take, as a provisional hypothesis, is that 
matter is just a collection of positive and negative units of electricity, 
