PRESIDENT’S ADDRESS. 13 
have been the gas in the vessel to begin with. If we pump out the gas 
until the pressure is too low to allow the discharge to pass, and then 
introduce a small quantity of gas and restart the discharge, the positive 
particles are the same whatever kind of gas may have been introduced. 
I have, for example, put into the exhausted vessel oxygen, 
argon, helium, the vapour of carbon tetrachloride, none of which contain 
hydrogen, and found the positive particles to be the same as when 
hydrogen was introduced. 
Some experiments made lately by Wellisch, in the Cavendish Labora- 
tory, strongly support the view that there is a definite unit of positive 
electricity independent of the gas from which it is derived; these experi- 
ments were on the velocity with which positive particles move through 
mixed gases. If we have a mixture of methyl-iodide and hydrogen 
exposed to Réntgen rays, the effect of the rays on the methyl-iodide 
is so much greater than on the hydrogen that, even when the mixture 
contains only a small percentage of methyl-iodide, practically all the 
electricity comes from this gas, and not from the hydrogen. 
Now if the positive particles were merely the residue left when a 
corpuscle had been abstracted from the methyl-iodide, these particles 
would have the dimensions of a molecule of methyl-iodide; this is very 
large and heavy, and would therefore move more slowly 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 negative particles through the mixture were the same as 
the velocities through pure hydrogen, although in the one case the ions 
had originated from methyl-iodide and in the other from hydrogen; a 
similar result was obtained when carbon tetrachloride, or mercury 
methyl, was used instead of methyl-iodide. 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 particles to the influence 
of a magnet is very marked; though there are cases when the positive 
