I threw up my curtain next morning to find London settling down into a sea of fog. Already the Thames was wholly hidden, and the water side of the embankment showed only faint, twinkling lights, just on the point of complete extinguishment. The caped policeman, the hurrying butcher’s boy, the laborers and the charwomen passing through the garden below, had all completely lost their individuality and became, in place of common London types, misty twentieth century Niobes. But dismal though it was without, my spirits were cheerful enough within as I started down to meet Tom and Dorothy.

We were half through breakfast when Hamerly’s card was brought in, to be followed a few moments later by the man himself. I looked with delighted interest at the involuntary start that he gave when he met Dorothy. How I wish I might rightly describe her as she stood there, lighting by her very presence the gray interior of the dining-room, shrouded as it was by the “London particular.” Everything else was gloom, save in the circle where Dorothy gave the radiance of her presence. Hamerly’s silent tribute was no more than she exacted from all who met her. Again and again I marvelled at my audacity in believing I might have this incarnation of youth, of power, and beauty for my own.

Such thoughts raced through my head as I sat watching the swift interchange of question and answer between Tom, Dorothy and Hamerly. In response to their inquiries, Hamerly related the story he had told me the day before, and as he ended, asked, “What are you going to do next? How are you planning to use your man Swenton?”

Dorothy answered for Tom and myself. “We are going straight to Dr. Heidenmuller’s laboratory, taking Swenton along. I want to have the whole scene before my eyes to see what can be made out of it. We should be very glad to have you come with us, Mr. Hamerly.”

Tom bent towards me with a look of mock anguish on his brow. “How I had hoped for a peaceful Sunday morning,” he said, in a low aside, “and now we’ve got to plunge out into a nasty fog, and chase all over this benighted city. Never mind, I might have known. I never can have my own way.”

Despite his plaint, Tom was the first one ready, as, clothed in raincoat and slouch hat, our little party gathered under the shelter of the glass awning inside the court.

The massive dignity of the carriage porter, shrouded in a white glistening rubber coat, loomed bulkier than ever, as, with an elephantine grace, he whistled shrilly twice. Out of a dim background two hansoms dashed into the circle of light where the arcs of the entrance fought bravely against the encroaching fog.

“I’m going with Mr. Hamerly,” said Tom. “You take Dorothy in the other hansom, Jim, and drive straight. We’ll pick up Swenton on the way. Give the address, will you, Hamerly?”

“Old Jewry, third alley, this side of Gresham Street,” said Hamerly, and the cabbies nodded.

Dorothy stepped lightly in before I could lend my aid. I followed, the porter closed the curtaining doors, pulled up the window, and we were off, embarked on a sea of fog. As I looked out, I thought I saw Tom speaking to our driver, but I could not be sure.

“Old Jewry,” said Dorothy dreamily. “How delightfully Dickensonian. I haven’t an idea where it can be, and I don’t want to know. It’s much more fun plunging off into an unknown world of adventure in the good ship Hansom Cab.”

I happened to have a strong idea where the Old Jewry was, but some guardian angel kept me from speaking. Never before had I possessed all that was precious to me in life in the small capacity of a hansom cab. Outside passed slowly by a dim neutral city, into which street lamps cast pointed lines of light in a vain endeavor to pierce the gloom, where ghosts, appearing suddenly under our horses feet, disappeared quite as suddenly into the blanketing darkness, and where now and then a motor bus came looming past us, like some high-pooped caravel of Spain. Now and again we stopped. Now and again we crept at a foot pace through what seemed at one and the same time an eternity of joy and a fleeting moment of happiness. Dorothy lay back against the cushioned corner, taking in the experience to the utmost. We spoke but seldom. I proffered no suggestions. It was enough for me to sit beside her, to know the rough cloth of her tweed ulster touched my hand, to feel through every inmost fibre of my being her dear and sweet proximity. On and on we travelled, till at length I came to the sudden realization that, according to all my impressions, we should have been at our destination long before. I looked out carefully for the first time. The fog was as dense as ever. I knew nothing of my whereabouts. Saying no word to Dorothy, I kept on trying to pierce the wall of cloud, as a hundred questions began to spring up in my brain. Was there something queer in this? Was the driver lost, or was he purposely taking us in some dangerous direction? It did not matter, anyway. As I looked at Dorothy, I knew I could protect her against a thousand perils, and I felt a warm glow of power, of courage springing within my soul. Just then I saw some arc lights ahead, and I peered yet more carefully. Under them the fog seemed less dense, and when a brass plate showed I scanned it eagerly. “Charterhouse.” I could read no more, but that told me where I was. In Charterhouse Square, beyond Smithfield, almost to Clerkenwell Road. We had gone far out of our way, while I had been dreaming. I threw up the driver’s door. “You must be out of your way,” I cried.

“H’I couldn’t do better, sir,” came the answer. “I ’ad to come round, I’m ’eaded straight for the h’old Jewry, sir.”

Perhaps there was a note of laughter in the man’s voice, certainly there was nothing sinister. I recalled the glimpse I had caught of Tom beside the cab at the Savoy, and, my qualms ceasing, I inwardly blessed that mischievous spirit.

Dorothy looked up as I spoke. “Is it all right, Jim?” she asked.

“It is perfectly all right,” I answered, and she fell back into her happy meditation, while I inwardly made still more remarks on her ingenious brother. Silent and happy we went on, my mind quite at rest now, and not in the least anxious to come to the end. The cab stopped and the little door at the top opened with a click.

“This is the place, sir.”

I jumped out and looked around. No cab in sight. “Well,” I said to Dorothy, “here’s a pretty go. Nobody in sight, and I don’t know which is the house.”

Without a word, Dorothy leaned forward and whistled a single bar. Out of the fog came the notes repeated, and a moment later across the street came Tom.

“Oh, you’ve reached here finally, have you?” said he, a trace sarcastically. “I thought you’d never arrive; I couldn’t imagine what kept you.”

As he spoke, I heard a sort of choked gasp from the top of the hansom, but fortunately Dorothy’s suspicions were not aroused.

“It hasn’t seemed so very long,” she answered simply, to which Tom responded, “Oh, really, hasn’t it?” as he took her arm to lead her across the street. He called back to our cabby as we left, “Drive forward a little, and you’ll find a sort of shelter where you can wait. The other cab’s there.”

“Right, sir,” came the reply, and we heard the slow movement of his disappearing wheels, as we three were left in the ocean of fog.

“Swenton’s hunting up the caretaker,” said Tom. “Hamerly and I have been waiting for him to come back. The old rooms are locked up tight.”

We found Hamerly in a vestibule where a single gas lamp flickered, and, as we waited, we fell to talking in low tones. The mist seemed to bring our voices to a minor key. Perhaps ten minutes had passed, when the door opened, and Swenton entered, accompanied by a man in a coarse ticking apron.

“This is the caretaker, sir,” he began, bowing to Dorothy and me. “He refused to let me in to get my things. Says the laboratory was left after Dr. Heidenmuller’s death to another chemist, a gentlemen who bought all the doctor’s stuff from the heirs. He was there, off and on, for a little while, but he went away quite a long time ago,—went one night suddenly and never came back. This man says the agents won’t allow anybody in. I brought him here, so you could talk to him if you wished.”

The caretaker stood silent and sullen as Swenton spoke, his hands deep in the front pockets of his apron.

“I do want to speak with him,” said Tom briefly. “Come here,” and he led the way apart, the caretaker following. A moment’s conversation was broken only by a golden clink, accompanied by the jingle of keys, after which the caretaker disappeared, and Tom turned back to us.

“I have here,” he said mysteriously, “a bunch of keys which I strangely found on the floor in the rear of this hall. Suppose we ascend to the top floor and see if they will work there.”

Dorothy’s face was clouded as Tom came up to the spot where we were standing a little apart, Hamerly and Swenton had already started up the stairs. “I’m not sure that you are doing right in this, Tom,” said Dorothy swiftly, in a low voice. “I don’t like to bribe a servant to let us into a place where we don’t belong.”

Tom’s face became serious in a minute. “I don’t like it either, Dorothy,” he answered gravely, “but I’m going to do it. Do you remember the little German middy lying down at the bottom? As long as the man who is trying to stop all war is at large there are thousands of men in hourly peril. I honestly believe we are the only ones who can run the man down. I am convinced we shall be wholly justified in such action.”

Dorothy stood for a moment in silent thought. “I think you are right, Tom,” she said quietly. “In this case I hope and believe the end will justify the means. We must find ‘the man.’ Go ahead.”

Stumbling through the darkness, we reached the top, where the flame of a match showed a strong oak door with two Yale locks upon it. Tom had the keys in immediately and threw the door open. Once within, Swenton passed with accustomed step to the wall, turned a switch, and incandescents lighted the whole place.

We were in a sort of anteroom, with desks and chairs. “The outer office,” said Swenton briefly. We passed through an inner door. “The main laboratory,” remarked Swenton. This was similar to any other laboratory. A good sized motor generator in one corner, covered by a rubber sheet, a couple of tile-topped tables, a set of shelves on one side, filled with labelled reagent bottles, a set of glass cases, supported on a base filled with drawers, on the other. In the cabinets were glass ware and apparatus of various sorts. Tom started for the case, but Dorothy laid a restraining hand on his arm.

“Wait till we have seen it all. Then we’ll go over the whole, piece by piece.”

Tom nodded, and we went on. There were three doors on the opposite side of the wall. Swenton passed to the first and opened it. “The storeroom,” he explained. Within were wooden cases of glassware, large carboys of acid, glass tubing on racks and wire on spools. In one corner was apparently a hospital for broken or disused pieces of apparatus. We turned from this to the second door. “The balance room,” said Swenton, as he threw open the portal. Three balances in polished wood and shining glass met our eyes. There was nothing else in the room. Swenton opened a third door. “The spectroscope room,” he said. “Beyond is the doctor’s private laboratory.” A big piece of apparatus on the table was covered with a green cloth. Beyond was a wooden door. Despite myself, I felt a queer, nervous tremor pass over my frame, as I looked at the commonplace wooden panels, behind which Dr. Heidenmuller had sat dead, killed by the same mysterious power which had slain the men I had seen lying quietly at the bottom of Portsmouth Harbor. Tom and Hamerly were as keen as hounds on a scent, Swenton interested but more indifferent, Dorothy pale, her eyes glittering with excitement. Hamerly reached the door first, tried it and it swung back. The incandescent had not been turned on in the spectroscope room, and the only light which entered was the golden lane, which came through from the main laboratory. It seemed like a stage setting. The light fell on a heavy wooden table and a couple of Windsor chairs. The rest was but faintly outlined.

A moment’s pause on the threshold, as if we expected to meet some horror, we knew not what, and then we rushed in together. There was nothing to be seen. Wood panelled walls; windows sealed by wooden shutters; the wooden table and the two wooden chairs; that was all. We stood there silent, until Tom broke the quiet.

“Nothing to do but to Sherlock Holmes it,” he said. “We have all day to run this thing down. Swenton, there’s a piece of apparatus here that I need. The doctor may never have had it outside his room as a whole, yet we may find traces of it in the laboratory or the storeroom. Are you willing to help us hunt?”

“I should be the most ungrateful man living, sir, if I were not,” said Swenton earnestly. “I owe my wife’s life to you and Miss Haldane.” He glanced at Dorothy.

“So that’s where you have been the last two mornings,” I whispered to her, as Tom went on.

“I found them just coming out of great distress,” she answered simply; “I am so glad I was able to help.”

“Now,” cried Tom, “let’s sit down to another counsel of war. Come out into the outer laboratory and we’ll talk it all over.”

The drawn shades, the bright gleam of the laboratory lamps reflected back from polished tile and cabinet door, gave a distinctly cheerful aspect to the scene as we settled down.

“I have been thinking this matter over carefully for some time,” began Hamerly, in his rather careful tones, once we were seated, “and if you do not object I should like to present my theories.”

“Go right ahead,” said Tom.

Hamerly went on somewhat thoughtfully. “I think you are wrong in saying we ought to follow the methods of Sherlock Holmes. We ought rather to follow Dupin, Poe’s detective, the man who preceded Sherlock Holmes. Try to reason out what the doctor would have on hand with regard to the power, and where he would have it. Try to analyze the action of his brain, rather than hunt for minute data. Let’s see what we know about Dr. Heidenmuller. He was a German of the most typical student type. That means he would never do anything without putting it down on paper. He had every desire to keep what he was doing from those around him. That is evident from the fact that Swenton never knew anything about the interior of this room. If the doctor made notes, as I believe he must have done, he would have wanted them within reach. So he must have had them in this room. He was a brilliant scientist, therefore he would not by preference have used any of the ordinary methods of concealment. His notes and apparatus were likely to take up a comparatively large amount of space, so that we are impelled to the definite conclusion that there is a concealed closet somewhere in that inner chamber. If we could take the time to remove the whole of the walls, and could get permission to do so, we could, I believe, find the hiding place, but that would involve time, expense, and running down the people who at present control the place and own the apparatus. I strongly question whether that would be worth while.”

“No,” said Tom, “I don’t believe it would. If there were any chance of the man who has hired this place being the man we are after, I’d say go for him at any cost, but I don’t believe there’s one chance in a thousand that it is. He’s too sharp to stay around where Dr. Heidenmuller died under such peculiar circumstances.”

“I agree to that,” said Hamerly.

“And I, too,” I chimed in.

Dorothy said nothing, but as I watched her, I saw the rose of her cheeks growing deeper, and that peculiar change in her eye that showed she had already leaped beyond the reasoning of the others and grasped the answer by intuition. “One question first,” she began, “Mr. Swenton, did the doctor leave the door to the spectroscope room open when he went into his private room?”

“No,” answered Swenton slowly, “he would go into the spectroscope room, lock that door, and then you could hear the inner door open and shut. Sometimes he would not come out again, but I have often heard him come out into the anteroom about three or four minutes after he went in, stay there for a minute or two, then go in again and come out once more. After that he would be shut up there for hours together.”

“That settles it,” cried Dorothy. “I’m sure I know how he opened his secret closet or closets. You remember the insulated wire covering they found, when they came in after the doctor’s death.”

We nodded eagerly.

“That was the winding of an electro-magnet. He attached it to the long flexible cord of that incandescent light socket in the anteroom, took it in, opened his closets, brought it out again, and went back. See if you can find an electro-magnet in the cases or the storeroom, and we’ll open things up.”

Scarcely were the words out of her mouth, before Swenton had hurried to a drawer, and pulled out three small electro-magnets, all of the same size.

“Here are the only ones I know of, in the laboratory,” he exclaimed. “I can connect one of them with the flexible cord in a minute. We shall want more light, though. If one of you gentlemen will get another connector and fix it to a socket, I’ll fit the magnet. You’ll find some connectors for that size socket in the storeroom, I’m sure.”

With four practised hands at work, it was scarce ten minutes before an incandescent stood on the table in the inner room, while we had an electric magnet connected to a long flexible cord which brought current from an incandescent light socket in the next room. Dorothy stood in the centre, once more in command.

“I believe it’s under one of those pegs,” she said. “See what’s under them.”

Round and round the room we went, pulling at every peg that joined the sealed walls. Under each was a nail. Tom picked up one of the pegs as we drew it forth.

“Humph!” he cried. “Insulated by caema. That explains why the nails were left. What a careful job this was, anyway.”

Hamerly and Swenton nodded. I started to ask what caema was, but I was pulling on a particularly refractory peg just then and let it go. The word stuck fast in my memory, however. It was the same one I had seen in Tom’s book on our journey up from Portsmouth. As each peg came out, the little electro-magnet was brought up to the hole and its action watched. Not a nail stirred. We had gone around three sides of the room, when Tom called out, “This peg came easily. Bring over the magnet.”

Before I could bring the magnet within an inch of the hole, the nail within sprang out and attached itself to the magnet, just as a needle springs up and clings to the horseshoe magnet of a child. As it sprung, the whole panel, four feet high and three feet across, opened on easy hinges and swung outward, showing a small inner door. Tom gave a long, low whistle. “Right again, sister,” he remarked. “What should we do without you?”

The stout oak door, strong as it was, proved no obstacle to our attack, and readily swung outward. Stooping, we peered within. Empty shelves on one side. A row of drawers on the other. One by one we drew the drawers from their places. Every one was empty. From top to bottom of the recess we searched, but without avail. Finally we straightened up with blank faces.

“There must have been something there,” said Dorothy slowly.

“Hang it,” ejaculated Tom, “I know there was. If you want to know my real opinion, there has been somebody here ahead of us. I don’t believe we’ll find a thing.”

We did not, and the last inspection over, we were ready to take our leave, when Tom broke in.

“One last thing,” he said; “I want to see how that incandescent light in the ceiling can be connected without outside metal. That reflector, by the way, looks like clear glass, but it must have some reflecting power.”

He jumped lightly to a chair, thence to the table, and turned to look through the clear glass of the big hemispherical shade, which had guarded the incandescent in the ceiling.

“Oh, I say,” he exclaimed, “here’s a most extraordinary thing. Everything seen through this is bent double. Here’s the biggest refraction I ever saw. Can it be the glass, or something inside of it? This thing is hermetically sealed above. Do you know, I believe we’ve got one solution of the mystery here.”

We all stood looking eagerly up at him, as he gazed through the globe.