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to be interpreted in terms of further unknown things, the number 
of unknowns being thereby reduced. So matter was coming to 
be explained in terms of electricity, itself an unknown. The falling 
of bodies had long been explained as due to gravity; but gravity 
was unexplained. One of the fundamental properties of matter— 
inertia, that property in virtue of which force and time were 
required to set it in motion or to bring it to rest—was now believed 
to be identical with magnetic inertia. The lecturer illustrated the 
principle of inertia by means of a heavy gyroscope. He then 
showed that when an electric current is passed through a vacuum 
tube, particles negatively charged are projected at great velocity 
from the negative pole or kathode, a luminosity being caused at 
the surface of the glass where they strike. That these particles 
actually travel through the tube was shown by the shadow cast 
by an object interposed in their path. These kathode rays of 
Crookes have been shown by Sir Joseph J. Thomson to consist 
of particles far smaller than atoms, and have been called by him » 
electrons. Any form of matter is capable of yielding electrons. 
By a number of experiments the lecturer showed that when a 
magnet is brought near the stream of electrons, the stream is 
deflected in a direction at right angles to the direction of the magnetic 
force, much as the gyroscope was deflected at right angles to a 
force impressed upon it. An electric current, in fact, is a magnet, 
and the energy required to produce a current is the energy required 
to produce the magnetic field of force. 
The hypothesis that matter consists of atoms has been found 
necessary to explain the chemical properties of matter. It is now 
found that atoms consist wholly, or in part, of smaller particles— 
electrons, which appear to be units of negative electricity. When 
the particles of a body are set in vibration by heat, light is emitted. 
Now light is known to be an electric vibration, and there seems 
every reason to suppose that light is due to the vibrations of 
electrons. 
An hypothetical constitution of the atom, on the assumption 
that it consists of electrons, has been propounded by Sir J. J. 
Thomson, which goes some way to account for the periodic law 
of chemistry. When the atoms of the chemical elements are 
arranged in the order of their weight, it is well known that they 
fall into groups or periods, at corresponding phases of which occur 
atoms having similar chemical properties. Thomson supposes an 
atom to consist of a sphere of positive electricity, in the interior 
of which are a definite number of negative electrons. The latter 
will necessarily be attracted towards the centre of the positive 
