led between the outer and inner shells of crystal
walls; lifts that shot smoothly to the laboratories and
pent-apartments on the roofs of the two structures. For Simeon Pullman
had been a close friend of Robbins; a fellow physicist, in fact. They
interchanged theories, results of experiments, and found this swift
connection most convenient.

Both men were dead now--Pullman as the result of a premature
explosion, and Robbins, executed by the Mercutians. But the secret
passageway remained.

Hilary pressed the secret spring he knew of. A gleaming oblong of
crystal slid silently open. He went in without hesitation and the
slide closed with a little whir behind him.

A low tunnel confronted him, just barely high enough for him to move
without stooping. The walls here were of burnished metal, glowing with
impregnated cold-light. It was empty, silent. Evidently it had been
undisturbed for years. The Mercutians had not discovered this secret
way then.

* * * * *

The tunnel slanted downward for several hundred yards, then turned
sharply upward until a vita-crystal wall barred the way. Hilary could
hear vague sounds from the other side. He was in the Robbins
Building. He turned to the left, where a shaft stretched upward,
completely enclosed by crystal walls. A thin oblong edging showed the
platform beneath. He stepped on it, hesitated for a moment. There were
two control buttons; one that stopped the lift in the laboratory, the
other in the sleeping room that once was Martin Robbins'.

Hilary decided in favor of the penthouse; there was less chance of a
present occupant of the room. If there was--he shrugged his shoulders
and loosened the automatic in his blouse. He pressed the button.

The platform shot smoothly upward, up, up, thrusting a thousand feet
up. At length it came to a gliding halt. Hilary knew he was on the
roof now, in the interior of the wall making one side of the
sleep-apartment. The vita-crystal gleamed mockingly opaque at him. If
only he could see through; if only he had a Mercutian search beam now.
Was there someone in the room on the other side of the wall? He
strained his ears to listen, but the crystal was pretty much
sound-proof.

Very quietly Hilary drew his gun, broke it, examined the chamber. The
six bullets lay snug. He snapped it back in position, held the
automatic butt against his side, reached over and pressed the release
button.

* *