Stanford Beale spent a thoughtful three minutes in the darkness of the cellar passage to which Hilda Glaum had led him and then he began a careful search of his pockets. He carried a little silver cigar-lighter, which had fortunately been charged with petrol that afternoon, and this afforded him a beam of adequate means to take note of his surroundings.

The space between the two locked doors was ten feet, the width of the passage three, the height about seven feet. The roof, as he had already noted, was vaulted. Now he saw that along the centre ran a strip of beading. There had evidently been an electric light installation here, probably before the new owners took possession, for at intervals was a socket for an electric bulb. The new occupants had covered these and the rest of the wall with whitewash, and yet the beading and the electric fittings looked comparatively new. One wall, that on his left as he had come in, revealed nothing under his close inspection, but on the right wall, midway between the two doors, there had been a notice painted in white letters on a black background, and this showed faintly through the thick coating of distemper which had been applied. He damped a handkerchief with his tongue and rubbed away some of the whitewash where the letters were least legible and read:

This was evidently half an inscription which had been cut off exactly in the middle. To the left there was no sign of lettering. He puzzled the letters for a few moments before he came to an understanding.

"Air-raid shelter. Ambulance and first aid!" he read.

So that explained the new electric fittings. It was one of those underground cellars which had been ferreted out by the Municipality or the Government for the shelter of the people in the neighbourhood during air-raids in the Great War. Evidently there was extensive accommodation here, since this was also an ambulance post. Faintly discernible beneath the letters was a painted white hand which pointed downward. What had happened to the other half of the inscription? Obviously it had been painted on the door leading into the first-aid room and as obviously that door had been removed and had been bricked up. In the light of this discovery he made a more careful inspection of the wall to the left. For the space of four feet the brickwork was new. He tapped it. It sounded hollow. Pressing his back against the opposite wall to give him leverage he put his foot against the new brickwork and pushed.

He knew that the class of workmanship which was put into this kind of job was not of the best, that only one layer of brick was applied, and it was a mechanical fact that pressure applied to the centre of new work would produce a collapse.

At the first push he felt the wall sag. Releasing his pressure it came back. This time he put both feet against the wall and bracing his shoulders he put every ounce of strength in his body into a mighty heave. The next second he was lying on his back. The greater part of the wall had collapsed. He was curious enough to examine the work he had demolished. It had evidently been done by amateurs, and the whitewash which had been thickly applied to the passage was explained.

A current of fresh air came to meet him as he stepped gingerly across the debris. A flight of six stone steps led down to a small room containing a sink and a water supply, two camp beds which had evidently been part of the ambulance equipment and which the new owners had not thought necessary to remove, and a broken chair. The room was still littered with the paraphernalia of first-aid. He found odd ends of bandages, empty medicine bottles and a broken glass measure on the shelf above the sink.

What interested him more was a door which he had not dared to hope he would find. It was bolted on his side, and when he had slid this back he discovered to his relief that it was not locked. He opened it carefully, first extinguishing his light. Beyond the door was darkness and he snapped back the light again. The room led to another, likewise empty. There were a number of shelves, a few old wine-bins, a score of empty bottles, but nothing else. At the far corner was yet another door, also bolted on the inside. Evidently van Heerden did not intend this part of the vault to be used.

He looked at the lock and found it was broken. He must be approaching the main workroom in this new factory, and it was necessary to proceed with caution. He took out his revolver, spun the cylinder and thrust it under his waistcoat, the butt ready to his hand. The drawing of the bolts was a long business. He could not afford to risk detection at this hour, and could only move them by a fraction of an inch at a time. Presently his work was done and he pulled the door cautiously.

Instantly there appeared between door and jamb a bright green line of light. He dare not move it any farther, for he heard now the shuffle of feet, and occasionally the sound of hollow voices, muffled and indistinguishable. In that light the opening of the door would be seen, perhaps by a dozen pair of eyes. For all he knew every man in that room might be facing his way. He had expected to hear the noise of machinery, but beyond the strangled voices, occasionally the click of glass against glass and the shuff-shuff-shuff of slippered feet crossing the floor, he heard nothing.

He pulled the door another quarter of an inch and glued his eye to the crack. At this angle he could only see one of the walls of the big vault and the end of a long vapour-lamp which stood in one of the cornices and which supplied the ghastly light. But presently he saw something which filled him with hope. Against the wall was a high shadow which even the overhead lamp did not wholly neutralize. It was an irregular shadow such as a stack of boxes might make, and it occurred to him that perhaps beyond his range of vision there was a barricade of empty cases which hid the door from the rest of the room.

He spent nearly three-quarters of an hour taking a bearing based upon the problematical position of the lights, the height and density of the box screen and then boldly and rapidly opened the door, stepped through and closed it behind him. His calculations had been accurate. He found himself in a room, the extent of which he could only conjecture. What, however, interested him mostly was the accuracy of his calculation that the door was hidden. An "L"-shaped stack of crates was piled within two feet of the ceiling, and formed a little lobby to anybody entering the vault the way Beale had come. They were stacked neatly and methodically, and with the exception of two larger packing-cases which formed the "corner stone" the barrier was made of a large number of small boxes about ten inches square.

There was a small step-ladder, evidently used by the person whose business it was to keep this stack in order. Beale lifted it noiselessly, planted it against the corner and mounted cautiously.

He saw a large, broad chamber, its groined roof supported by six squat stone pillars. Light came not only from mercurial lamps affixed to the ceiling, but from others suspended above the three rows of benches which ran the length of the room.

Mercurial lamps do not give a green light, as he knew, but a violet light, and the green effect was produced by shades of something which Beale thought was yellow silk, but which he afterwards discovered was tinted mica.

At intervals along the benches sat white-clad figures, their faces hidden behind rubber masks, their hands covered with gloves. In front of each man was a small microscope under a glass shade, a pair of balances and a rack filled with shallow porcelain trays. Evidently the work on which they were engaged did not endanger their eyesight, for the eye-pieces in the masks were innocent of protective covering, a circumstance which added to the hideous animal-like appearance of the men. They all looked alike in their uniform garb, but one figure alone Beale recognized. There was no mistaking the stumpy form and the big head of the Herr Professor, whose appearance in Oliva Cresswell's room had so terrified that young lady.

He had expected to see him, for he knew that this old German, poverty-stricken and ill-favoured, had been roped in by van 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. He was on the point of lifting down a box to make a more thorough examination when he heard a quavering voice beneath him.

"What you do here—eh?"

Under the step-ladder was one of the workers who had slipped noiselessly round the corner of the pile and now stood, grotesque and menacing, his uncovered eyes glowering at the intruder, the black barrel of his Browning pistol covering the detective's heart.

"Don't shoot, colonel," said Beale softly. "I'll come down."