old into a warm body, and thus transgress the principle
of Carnot.

Sec. 5. THE X RAYS

It appears to-day quite probable that the X rays should be classed
among the phenomena which have their seat in the luminous ether.
Doubtless it is not necessary to recall here how, in December 1895,
Roentgen, having wrapped in black paper a Crookes tube in action,
observed that a fluorescent platinocyanide of barium screen placed in
the neighbourhood, had become visible in the dark, and that a
photographic plate had received an impress. The rays which come from
the tube, in conditions now well known, are not deviated by a magnet,
and, as M. Curie and M. Sagnac have conclusively shown, they carry no
electric charge. They are subject to neither reflection nor
refraction, and very precise and very ingenious measurements by M.
Gouy have shown that, in their case, the refraction index of the
various bodies cannot be more than a millionth removed from unity.

We knew from the outset that there existed various X rays differing
from each other as, for instance, the colours of the spectrum, and
these are distinguished from each other by their unequal power of
passing through substances. M. Sagnac, particularly, has shown that
there can be obtained a gradually decreasing scale of more or less
absorbable rays, so that the greater part of their photographic action
is stopped by a simple sheet of black paper. These rays figure among
the secondary rays discovered, as is known, by this ingenious
physicist. The X rays falling on matter are thus subjected to
transformations which may be compared to those which the phenomena of
luminescence produce on the ultra-violet rays.

M. Benoist has founded on the transparency of matter to the rays a
sure and practical method of allowing them to be distinguished, and
has thus been enabled to define a specific character analogous to the
colour of the rays of light. It is probable also that the different
rays do not transport individually the same quantity of energy. We
have not yet obtained on this point precise results, but it is roughly
known, since the experiments of MM. Rutherford and M'Clung, what
quantity of energy corresponds to a pencil of X rays. These physicists
have found that this quantity would be, on an average, five hundred
times larger than that brought by an analogous pencil of solar light
to the surface of the earth. What is the nature of this energy? The
question does not appear to hav