powder instantly changes to
the gaseous condition."

"By pressing this lever"--Evans pointed at the box--"a vacuum is
created. Instantly the powder becomes a gas, which shoots forth
through this aperture with the speed of a projectile, taking the form
of a beam of absolute blackness. Or it can be discharged from
cylinders in such a way as to extend over a large area within a few
minutes."

"But how does this darkness make the invisible airships luminous?"
asked Stopford. "Why does not your darkness destroy all light?"

"In this way, sir," replied the old inventor. "The shadow-breaking gas
with which the airships are painted confers invisibility because it
absorbs sunlight. But it does not absorb the still more rapid waves,
or oscillations which manifest themselves as radio-activity. On the
contrary, it gathers and reflects these.

"Now Roentgen, the discoverer of the X-ray, observed that if X-rays
are allowed to enter the eye of an observer who is in complete
darkness, the retina receives a stimulus, and light is perceived, due
to the fluorescent action of the X-rays upon the eyeball.

"Consequently, by creating a beam of complete darkness, I bring into
clear visibility the fluorescent gas that coats the airships; in other
words, the airships become visible."

"If a light ray is nullified upon entering the field of darkness, will
it emerge at the other edge as a perfect light ray again?" asked
Stopford.

"It will emerge unchanged, since the black beam destroys light by
slightly slowing down the vibrations to a point where they are not
perceived as light by the human eye. On emerging from the beam,
however, these vibrations immediately resume their natural frequency.
To give you a homely parallel, the telephone changes sound waves to
electric waves, and re-converts them into sound waves at the other
end, without any appreciable interruption."

"Then," said Stopford, "the logical application of your method is to
plunge every city in the land into darkness by means of this gas?"

"That is so, sir, and then we shall have the advantage of
invisibility, and the enemy ships will be in fluorescence."

"Damned impracticable!" muttered Stopford.

"You seriously propose to darken the greater part of eastern North
America?" asked the Secretary for War.

"The gas can be produced in large quantities from coal tar besides
existing in crystalline deposits," replied Luke Evans. "It is so
volatile that I estimate that a