lfed the battery and a score of men.

Then suddenly Dick understood. He flung home the soaring lever,
banked, and headed, not for the White House, but for the flat roof of
the hotel from which the black searchlight was still projecting itself
through the skies. He hovered above, and dropped, light as a feather,
upon the rooftop.

* * * * *

There was only one person there--an old man dressed in a shabby suit,
kneeling before a great block of stone that had been dislodged upward
from the parapet and formed a sort of table. Upon this table the old
man had placed a large, square box, resembling an exaggerated kodak,
and it was from the lens of this box that the black beam was
projecting.

Dick sprang from his cockpit as the old man rose in alarm. He ran to
him and caught him by the arm.

"Luke Evans!" he cried. "Thank God you've come back in time to save
America!"

CHAPTER VI

_The Gas_

In the Blue Room of the White House the Council listened to old Luke
Evans's exposition of his invention with feelings ranging from
incredulity to hope.

"I've been at work all the time," said the old man, "not far from
here. I knew the day would come when you'd need me. I put my pride
aside for the sake of my country."

"Tell us in a few words about this discovery of yours, Mr. Evans,"
said Colonel Stopford.

Luke Evans placed the square black case upon the table. "It's simple,
like all big things, sir," he answered. "The original shadow-breaking
device that I invented was a heavy, inert gas, invisible, but almost
as viscous as paint. Applied to textiles, to inorganic matter, to
animal bodies, it adheres for hours. Its property is to render such
substances invisible by absorbing all the visible light rays that fall
upon it, from red to violet. Light passes through all substances that
are coated with this paint as if they did not exist."

"And this antidote of yours?" asked Colonel Stopford.

"Darkness," replied Luke Evans. "A beam of darkness that means
absolute invisibility. It can be shot from this apparatus"--he
indicated the box upon the table. "This box contains a minute portion
of a gas which exists in nature in the form of a black, crystalline
powder. The peculiar property of this powder is that it is the
solidified form of a gas more volatile than any that is known. So
volatile is it that, when the ordinary atmospheric pressure of fifteen
pounds to the square inch is removed, the