n Heerden, and
Beale, who pitied the old man, had been engaged for a fortnight in
trying to worm from the ex-professor of chemistry at the University of
Heidelberg the location of van Heerden's secret laboratory. His efforts
had been unsuccessful. There was a streak of loyalty in the old man,
which had excited an irritable admiration in the detective but had
produced nothing more.

Beale's eyes followed the benches and took in every detail. Some of the
men were evidently engaged in tests, and remained all the time with
their eyes glued to their microscopes. Others were looking into their
porcelain trays and stirring the contents with glass rods, now and again
transferring something to a glass slide which was placed on the
microscope and earnestly examined.

Beale was conscious of a faint musty odour permeating the air, an
indescribable earthy smell with a tang to it which made the delicate
membrane of the nostrils smart and ache. He tied his handkerchief over
his nose and mouth before he took another peep. Only part of the room
was visible from his post of observation. What was going on immediately
beneath the far side of the screen he could only conjecture. But he saw
enough to convince him that this was the principal factory, from whence
van Heerden was distilling the poison with which he planned humanity's
death.

Some of the workers were filling and sealing small test-tubes with the
contents of dishes. These tubes were extraordinarily delicate of
structure, and Beale saw at least three crumble and shiver in the hands
of the fillers.

Every bench held a hundred or so of these tubes and a covered gas-jet
for heating the wax. The work went on methodically, with very little
conversation between the masked figures (he saw that the masks covered
the heads of the chemists so that not a vestige of hair showed), and
only occasionally did one of them leave his seat and disappear through a
door at the far end of the room, which apparently led to a canteen.

Evidently the fumes against which they were protected were not virulent,
for some of the men stripped their masks as soon as they left their
benches.

For half an hour he watched, and in the course of that time saw the
process of filling the small boxes which formed his barrier and
hiding-place with the sealed tubes. He observed the care with which the
fragile tubes were placed in their beds of cotton wool, and had a
glimpse of the lined interior of one of the boxes