nd in the doing of which it avoids the oxidizing action of the
forging heat. Instead of heating the pieces to be welded in a forge, as
is now done, the ends to be united are simply brought into contact, and
the current is sent through the ends until they are in a soft condition,
after which the parts are pressed together and united by the simple
merging of the plastic condition in which they are reduced by the high
electric heat.

This form of welding makes the most perfect joint, and requires no
hammering, as the mass of the metal flows from one part or end to the
other; the unity is a perfect one, and the advantage is that the metals
can be kept in a semi-fluid state for a considerable time, thus assuring
a perfect admixture of the two parts.

With the ordinary form of welding it is necessary to drive the heated
parts together without any delay, and at the least cooling must be
reheated, or the joint will not be perfect.

The smallest kinds of electric heating apparatus are now being made, so
that small articles, sheet metal, small rods, and like parts can be
united with the greatest facility.

CHAPTER XVIII

X-RAY, RADIUM, AND THE LIKE

The camera sees things invisible to the human eye. Its most effective
work is done with beams which are beyond human perception. The
photographer uses the _Actinic_ rays. Ordinary light is composed of the
seven primary colors, of which the lowest in the scale is the red, and
the highest to violet.

Those below the red are called the Infra-red, and they are the Hertzian
waves, or those used in wireless telegraphy. Those above the violet are
called Ultra-violet, and these are employed for X-ray work. The former
are produced by the high tension electric apparatus, which we have
described in the chapter relating to wireless telegraphy; and the
latter, called also the Roentgen rays, are generated by the Crookes'
Tube.

This is a tube from which all the atmosphere has been extracted so that
it is a practical vacuum. Within this are placed electrodes so as to
divert the action of the electrical discharge in a particular direction,
and this light, when discharged, is of such a peculiar character that
its discovery made a sensation in the scientific world.

The reason for this great wonder was not in the fact that it projected a
light, but because of its character. Ordinary light, as we see it with
the eye, is capable of being reflected, as when we look into a mirror at
an