the jew silently and promptly set out in the wake of the hurrying woman; presently she and her pursuer disappeared round a corner.

"that's the result of our call, mapperley!" said hetherwick. "she's gone somewhere—to tell somebody!"

"likely!" assented mapperley. "but wherever she's gone, issy goldmark'll spot her. he's the eyes of a lynx."

"he let baseverie slip him, the other night, though," remarked hetherwick.

"well, there was some excuse for that," said mapperley, "to begin with, he was only instructed to find out where baseverie went, and to end with he had found out! he'll not let this woman slip him. she's good to follow—plenty of her."

"i wish we knew what she'd left in that house," said hetherwick. "we'll have to find out, somehow!"

"that's a police job," replied mapperley. "can't walk into people's houses without a warrant. and you say matherfield's on the other track? however, i should say that this woman's gone off now to find somebody who's principally concerned—she looked afraid, in my opinion, when she saw me."

"she's in it, somehow," muttered hetherwick.

"that house looks mysterious enough for anything. we'll keep a close watch on it, anyway, until goldmark comes back, however long that may be."

but the jew was back within twenty minutes. so was the woman. she came first, hurrying up the street quicker than when she had left it. as far as the watchers could make out from their vantage point, twenty yards away from her door, she looked flustered, distressed, upset. after her, on the opposite pavement, came mr. issy goldmark, his hands in his pockets.

the woman re-entered the house; they heard the door bang. a moment later the jew turned into the entry in which hetherwick and mapperley stood, half hidden from the street. he smiled, inscrutably.

"thee her go back to her houth?" he asked. "well, i followed. i thaw where thee'th been, too."

"where, then?" demanded hetherwick, impatiently.

goldmark jerked his head in the direction from whence he had come.

"round that corner," he said, "you get into a regular thlum. little thtreeth, alleyth, pathageth, and tho on. in one of 'em, a narrow plathe, where there'th a thort of open-air market, there'th a good thithed pieth of blank wall, with an iron-fathen'd door in it. well, the woman went in there—let herthelf in with a key that thee took from her pocket. ath thoon ath thee'd gone in, i took a clother look. the door'th fathen'd with iron, or thteel, ath i thaid—jolly thtrong. there ain't no name on it, and no keyhole that you can look through. the wall'th a good nine or ten feet high, and it'th covered with broken glath at the top. not a nithe plathe to get into, nohow!"

"well?" inquired hetherwick. "she went in?"

"went in, ath i thay, mithter, and the door clothed on her. after i'd taken a glimpth at the door i got a potht behind one of the thtalls in the thtreet and watched. she came out again in about ten minitth—looked to me, too, ath if thee hadn't had a very plethant time inthide. upthet! and thee thet off back here, fathter than vhat thee came. now thee'th gone into her houth again—ath you no doubt thaw. and that'th all. but if i wath you, mithter," concluded issy, "i should jutht find out vhat there ith behind that door and the wall it'th thet in—i thhould tho!"

"that's a police job," said mapperley once more. "if we'd only got matherfield with us, we could——" hetherwick paused—thinking. "look here, mapperley," he continued, with a sudden inspiration. "i know what we'll do! you get a taxi-cab, as quickly as possible. drive to the police station where i usually meet matherfield. there's another man there whom i know, and who's pretty well up in this business—detective-sergeant robmore. ask for him. tell him what we've discovered, and ask him to come back with you and to bring another man if he thinks it necessary. now then, goldmark! tell mapperley exactly where this place is."

the jew pointed along the street to its first corner.

"round that corner," he said. "firtht turning to the right; then firtht to the left; then firtht to the right—that'th the thpot. lot'th o' little thtallth in it—a bithy, crowded plathe."

"didn't ye notice the name?" demanded mapperley, half scoldingly.

"to be thure i did!" grinned goldmark. "pencove thtreet. but it'th better to dethcribe it than to name it. and don't you go tellin' no tackthy-driver to drive you in there!—cauth' there ain't room!"

mapperley gave no answer to this piece of advice; he shot off in the direction of victoria street, and hetherwick turned to the jew.

"we'll go and have another look at this place, goldmark," he said. "but we'll go separately—as long as we're in this street, anyway. you stroll off to that first corner, and i'll join you."

he crossed the street when the jew had lounged away, and once more took a narrow look at the house into which the big woman had vanished. it was as close barred and curtained as ever; a veritable place of mystery. for a moment hetherwick doubted whether he ought to leave it unwatched. but the descriptions of the wall and door in pencove street had excited his imagination, and he went on, turned the corner, and rejoined goldmark. goldmark at once went in front, piloting him into a maze of unusually dirty and crowded streets, and finally into one, narrower than the rest, on each side of which were tent-like stalls whereon all manner of cheap wares were being offered for sale by raucous-voiced vendors. he saw at once that this was one of those open-air markets of which there are many in the poorer neighbourhoods of london, and wherein you can buy a sixpenny frying-pan as readily as a paper of fried fish, and a gay neckerchief alongside a damaged orange.

threading his way behind issy, and between the thronged stalls and the miserable shops that lined the pavement, hetherwick presently came to the piece of blank wall of which the jew had told him. the houses and shops round about were old and dilapidated, but the wall was either modern or had been rebuilt and strengthened. it stretched between two low houses, one used as a grocer's, the other as a hardware shop. in length, it was some thirty feet; in height, quite ten; its coping, as goldmark had said, was liberally embattled with broken glass. the door, set flush with the adjoining masonry, was a solid affair, faced with metal, newly painted, and the lock was evidently a patent one. a significant fact struck hetherwick at once—there was no sign of a bell and none of a knocker.

"you say the woman let herself in here?" he asked, as he and issy paused.

"that'th it, mithter hetherwick—let herthelf in," replied issy. "i thee her take the key from her pocket."

hetherwick glanced at the top of the wall.

"i wonder what's behind?" he muttered. "building of some sort, of course." he turned to a man whose stall stood just in front of the mysterious door, and who at that moment had no trade. "do you know anything about this place?" he asked. "do you know what's behind this wall? what building it is?"

the stall-keeper eyed hetherwick over, silently and carefully. deciding that he was an innocent person and not a policeman in plain clothes, he found his tongue.

"i don't, guv'nor!" he answered. "'aint a bloomin' notion! i been comin' here, or hereabouts, this three year or more, but i 'aint never seen behind that wall, nor in at that there doorway. s'elp me!"

"but i suppose you've seen people go in and come out of the door?" suggested hetherwick. "it must be used for something!"

"i reckon it is, guv'nor, but i don't call nobody to mind, though, to be sure, i see a woman come out of it a while ago—big, heavy-jawed woman, she was. but queer as it may seem, i don't call to mind ever seeing anybody else. you see, guv'nor, i comes here at about ten o'clock of a morning, and i packs up and 'ops it at five—if there's folks comes in and out o' that spot, it must be early in a morning and late at night, and so i shouldn't see 'em. but it's my belief this here wall and door is back premises to something—the front o' the place'll be on the other side."

"that's a good idea," said hetherwick, with a glance at goldmark. "let's go round."

but there was no going round. although they tried various alleys and passages and streets that ought to have been parallel to pencove street, they failed to find any place that could be a frontage to the mysterious wall and its close-set door. but the jew's alert faculties asserted themselves.

"we can thee vhat'th behind that vail, mithter, eathy enough if we get one o' them thop-keeperth oppothit to let uth go upthtairth to hith firtht floor," he said. "look right acroth the thtreet there, thtallth and all, into vhatever there ith. try that one," he went on, pointing to a greengrocer's establishment which faced the close-set door. "tell him we're doin' a bit o' land thurveyin'—which ith thtrue!"

hetherwick made his request—the greengrocer's lady showed him and goldmark upstairs into a bow-windowed parlour, one of those dismal apartments which are only used on sundays, for the purpose of adding more gloom to a gloomy day. she observed that there was a nice view both ways of the street, but hetherwick confined his inspection to the front. he saw across the wall easily enough, now. there was little to see. the wall bounded a yard, bounded on its left and right sides by the walls of the adjoining houses, and at its further extremity by a low, squat building of red brick, erected against the rear of a high, windowless wall beyond. from its mere aspect, it was impossible to tell what this squat, flat-roofed structure was used for. its door—closed—was visible; visible, too, were the windows on either side. but it was easy to see that they were obscured, as to their lower halves, by coats of dark paint. there was no sign over the building; no outward indication of its purpose. in the yard, however, were crates, boxes, and carboys in wicker cases; a curiously-shaped chimney, projecting from the roof above, suggested the presence of a furnace or forge beneath. and hetherwick, after another look, felt no doubt that he was gazing at the place to which hannaford had been taken, and where he had been skilfully poisoned.

goldmark suddenly nudged his arm, and nodded at the crowded street below.

"mapperley!" he whispered. "and two men with him!"

hetherwick, glancing in the direction indicated, saw robmore and another man, both in plain clothes, making their way down the street, between the stalls and the shops. with them, and in close conversation, was a uniformed constable. he turned to leave the room, but goldmark again touched his elbow.

"before we go, mithter," he said, "jutht take another glanth at that plathe oppothite, and it'ths thurroundin'th. i thee where we can get in! d'ye thee, mithter hetherwick, the wall between that yard and the next houth—the right-hand thide one—'ith fairly low at the far end. now, if the man in that houth would let uth go through to hith back-yard—vhat?"

"i see!" agreed hetherwick. "we'll try it. but robmore first—come along."

he slipped some silver into the hand of the green-grocer's lady, and went down to the street. a few brief explanations to the two detectives supplemented the information already given them by mapperley, and then robmore nodded at the constable who stood by, eagerly interested.

"we've been talking to him, mr. hetherwick," he said. "he's sometimes on day duty here, and sometimes he's on night. he says he's often wondered about this place, and it's a very queer thing that though he's known this district more than a year, he's never seen a soul go in or out of that door, and hasn't the least notion of what business, if it is a business, is carried on there!"

"never seen anything or anybody!" corroborated the constable. "at any time—day or night. when i first came on this beat, maybe fifteen months ago, that door had been newly set and painted, and the glass had just been stuck a-top of the wall. but it's a fact—i've never seen anybody go in or come out!"

"i propose to go in," said hetherwick. "i think we've abundant cause, knowing what we do. it may be that the two missing ladies are there. i've been having a look into the yard, and we could get into it easily by going through the grocer's shop there, on the right, and climbing the wall from his back premises. what do you say, robmore?"

"oh, i think so!" agreed robmore. "now we're on the job, we'll carry it through. better let me tackle the grocer, mr. hetherwick—i'll see him first and then call you in."

the other waited while robmore entered the shop and spoke with its owner. they saw him engaged in conversation for several minutes; then he came to the door and beckoned the rest to approach.

"that's all right," he said in an aside to hetherwick. "we can go through to his back-yard, and he'll lend us a step-ladder to get over the wall. but he's told me a bit—he knows the two men who have this place in the next yard, and there's no doubt at all, from his description of them, that one's ambrose and the other baseverie. he says they've had the place almost eighteen months, and he thinks they use it as a laboratory—chemicals, or something of that sort. but he says they're rarely seen—sometimes he's never seen them for days and even weeks together. usually they're there of a night—he's seen lights in the place at all hours of the night. well—come on!"

the posse of investigators filed through the dark little shop to a yard at its rear, the grocer's apprentice going in front with a step-ladder, which he planted against the intervening wall at its lowest point. one by one, the uniformed constable going first, the six men climbed and dropped over. but for their own presence, the place seemed deserted and lifeless. as hetherwick had observed from the greengrocer's parlour the windows were obscured by thick coats of paint; nevertheless, two or three of the men approached and tried to find places from which the paint had been scratched, in an effort to see what lay inside. but the constable, bolder and more direct, went straight to the entrance.

"door's open!" he exclaimed. "not even shut!" he pushed the door wide, and went into the building, the rest crowding after him. "hullo!" he shouted. "hullo!"

no answer came to the summons. the constable crossed the lobby in which they were all standing, and opened an inner door. and hetherwick saw at once that the grocer's surmise as to the purpose to which the place was put had been correct—this was a chemical laboratory, well equipped, too, with modern apparatus. but there was not a sign of life in it.

"nobody here, apparently," murmured one of the men. "flown!"

robmore went forward to another door, and opening it, revealed a room furnished as an office. there was a roll-top desk in it, and papers and documents lying there; he and hetherwick began to finger and examine them. and hetherwick suddenly saw something that made a link between this mysterious place and the house he had called at earlier in the afternoon. there, before his eyes, lay some of the azure-tinted notepaper which mapperley had traced with the embossed address on it of which the stationer had told.

"there's no doubt we've hit on the place at last, robmore," he said. "i wish we'd had matherfield here. but——"

before he could say more, a sudden shout came from goldmark, who, while the others were investigating the lower regions, had courageously, and alone, gone up the low staircase to the upper rooms.

"mithter!" he called. "mithter hetherwick! come up here—come up, all of you. here'th a man here, a-thittin' in a chair—and th'elp me if i don't believe he'th a thtiff 'un—dead!"