hich throw
them first in one direction and then in another. The passage of a
current is a sort of flow of these electrons in a determined
direction. This electric flow brings, however, no modification to the
material medium traversed, since every electron which disappears at
any point is replaced by another which appears at once, and in all
metals the electrons are identical.

This hypothesis leads us to anticipate certain facts which experience
confirms. Thus J.J. Thomson shows that if, in certain conditions, a
conductor is placed in a magnetic field, the ions have to describe an
epicycloid, and their journey is thus lengthened, while the electric
resistance must increase. If the field is in the direction of the
displacement, they describe helices round the lines of force and the
resistance is again augmented, but in different proportions. Various
experimenters have noted phenomena of this kind in different
substances.

For a long time it has been noticed that a relation exists between the
calorific and the electric conductivity; the relation of these two
conductivities is sensibly the same for all metals. The modern theory
tends to show simply that it must indeed be so. Calorific conductivity
is due, in fact, to an exchange of electrons between the hot and the
cold regions, the heated electrons having the greater velocity, and
consequently the more considerable energy. The calorific exchanges
then obey laws similar to those which govern electric exchanges; and
calculation even leads to the exact values which the measurements have
given.[31]

[Footnote 31: The whole of this argument is brilliantly set forth by
Professor Lorentz in a lecture delivered to the Electrotechnikerverein
at Berlin in December 1904, and reprinted, with additions, in the
_Archives Neerlandaises_ of 1906.--ED.]

In the same way Professor Hesehus has explained how contact
electrification is produced, by the tendency of bodies to equalise
their superficial properties by means of a transport of electrons, and
Mr Jeans has shown that we should discover the existence of the
well-known laws of distribution over conducting bodies in electrostatic
equilibrium. A metal can, in fact, be electrified, that is to say, may
possess an excess of positive or negative electrons which cannot
easily leave it in ordinary conditions. To cause them to do so would
need an appreciable amount of work, on account of the enormous
difference of the specific inductive c