the naturally radioactive elements are all in the group of those with
the highest atomic weight. This fact, seen together with the
characteristics of radioactivity, tells us that in such elements
gravity has so far got the upper hand of levity that the physical
substance is unable to persist as a spatially extended, coherent unit.
It therefore falls asunder, with the liberated levity drawn into the
process of dispersion. Seen thus, radioactivity becomes a symptom of
the earth's old age.

*

Before entering into a discussion of the question, which naturally
arises at this point, as to how levity and gravity by their two
possible ways of interaction - 'sulphurous' or 'saline' - determine the
properties of so-called positive and negative electricity, we shall
first study the third mode of generating electricity, namely, by
electromagnetic induction. Along this way we shall arrive at a picture
of the magnetic force which corresponds to the one already obtained of
electricity. This will then lead us to a joint study of the nature of
electric polarity and magnetic polarity.

The discovery of the phenomena we call electromagnetic depended on the
possibility of producing continuous electrical processes. This arose
with Volta's invention. When it became necessary to find a concept for
the process which takes place in an electric conductor between the
poles of a galvanic cell, the concept of the 'current', borrowed from
hydrodynamics, suggested itself. Ever since then it has been the rule
to speak of the existence of a current within an electric circuit; its
strength or intensity is measured in terms of a unit named in honour of
Ampere.