They have measured not only the pressure, but also the
energy of the radiation by means of a special bolometer. They have
thus arrived at numerical verifications which are entirely in
conformity with the calculations of Maxwell.
The existence of these pressures may be otherwise foreseen even apart
from the electromagnetic theory, by adding to the theory of
undulations the principles of thermodynamics. Bartoli, and more
recently Dr Larmor, have shown, in fact, that if these pressures did
not exist, it would be possible, without any other phenomenon, to pass
heat from a cold 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.
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