PRESIDENTIAL ADDRESS. 395 
and colliding with each other, and with the metallic atoms, like the molecules of 
a gas on the kinetic theory. If the mass of each carrier were 1/1700 of that of 
an atom of hydrogen, the velocity at 0° C. would be about sixty miles a second, 
and would be of the right order of magnitude to account for the observed values 
of the conductivities of good conductors, on the assumption that the number of 
negative corpuscles was the same as the number of positive metallic atoms, and 
that the mean free path of each corpuscle was of the same order as the distance 
between the atoms. The same hypothesis served to give a qualitative account of 
thermo-electric phenomena, such as the Peltier and Thomson effects, and of 
radiation and absorption of heat, though in a less satisfactory manner. When 
extended to give a consistent account of all the related phenomena, it would 
appear that the number of free corpuscles required is too large to be reconciled, 
for instance, with the observed values of the specific heat, on the assumption that 
each corpuscle possesses energy of translation equal to that of a gas molecule at the 
same temperature. 
Sir J. J. Thomson has accordingly proposed and discussed another possible 
theory of metallic conduction, in which the neutral electric doublets present in 
the metal are supposed to be continually interchanging corpuscles at a very high 
rate. Under ordinary conditions these interchanges take place indifferently in all 
directions, but under the action of an electric field the axes of the doublets are 
supposed to become more or less oriented, as in the Grotthus-chain hypothesis 
of electrolytic conduction, producing a general drift or current proportional to 
the field. This hypothesis, though fundamentally different from the preceding 
or more generally accepted view, appears to lead to practically the same relations, 
and is in some ways preferable, as suggesting possible explanations of diffi- 
culties encountered by the first theory in postulating so large a number of free 
negative corpuscles. On the other hand, the second theory requires that each 
neutral doublet should be continually ejecting corpuscles at the rate of about 
1075 per second. There are probably elements of truth in both theories, but, 
without insisting too much on the exact details of the process, we may at least 
assert with some confidence that the corpuscles of caloric which constitute a 
current of heat in a metal are very closely related to the corpuscles of electricity, 
and have an equal right to be regarded as constituting a material fluid possessing 
an objective physical existence. 
If I may be allowed to speculate a little on my own account (as we are all here 
together in holiday mood, and you will not take anything I may say too seriously), 
I should prefer to regard the molecules of caloric, not as being identical with the 
corpuscles of negative electricity, but as being neutral doublets formed by the 
union Of a positive and negative corpuscle, in much the same way as a molecule 
of hydrogen is formed by the union of two atoms. Nothing smaller than a 
hydrogen atom has yet, so far as I know, been discovered with a positive charge. 
This may be merely a consequence of the limitations of our experimental methods, 
which compel us to employ metals to so large an extent as electrodes. In the 
symmetry of nature it is almost inconceivable that the positive corpuscle should 
not exist, if only as the other end of the Faraday-tube or vortex-filament repre- 
senting a chemical bond. Professor Bragg has identified the X or y rays with neu- 
tral corpuscles travelling at a high velocity, and has maintained this hypothesis 
with brilliant success against the older view that these rays are not separate 
entities, but merely thin, spreading pulses in the ether produced by the collisions 
of corpuscles with matter. I must leave him to summarise the evidence, but if 
neutral corpuscles exist, or can be generated in any way, it should certainly be 
much easier to detach a neutral corpuscle from a material atom or molecule than 
to detach a corpuscle with a negative charge from the positive atom with which 
it is associated. We should therefore expect neutral corpuscles to be of such 
exceedingly common and universal occurrence that their very existence might be 
overlooked, unless they happened to be travelling at such exceptionally high 
velocities as are associated with the y rays. According to the pulse theory, it is 
assumed that all y rays travel with the velocity of light, and that the enormous 
variations observed in their penetrative power depend simply on the thickness 
of the pulse transmitted. On the corpuscular theory, the penetrative power, like 
that of the a and B rays, is a question of size, velocity, and electric charge. 
Particles carrying electric charges, like the a and B rays, lose energy in producing 
ions by their electric field, perhaps without actual collision. Neutral or y rays do 
