Denison had scarcely gone to arrange for some one to watch the office that night, when Kennedy, having gathered up his radioscope and packed into a parcel a few other things from various cabinets, announced: "Walter, I must see that Miss Wallace, right away. Denison has already given me her address. Call a cab while I finish clearing up here. I don't like the looks of this thing, even if Haughton does neglect it."

We found Miss Wallace at a modest boarding-house in an old but still respectable part of the city. She was a very pretty girl, of the slender type, rather a business woman than one given much to amusement. She had been ill and was still ill. That was evident from the solicitous way in which the motherly landlady scrutinized two strange callers.

Kennedy presented a card from Denison, and she came down to the parlor to see us.

"Miss Wallace," began Kennedy, "I know it is almost cruel to trouble you when you are not feeling like office work, but since the robbery of the safe at Pittsburgh, there have been threats of a robbery of the New York office."

She started involuntarily, and it was evident, I thought, that she was in a very high-strung state.

"Oh," she cried, "why, the loss means ruin to Mr. Denison!"

There were genuine tears in her eyes as she said it.

"I thought you would be willing to aid us," pursued Kennedy sympathetically. "Now, for one thing, I want to be perfectly sure just how much radium the Corporation owns, or rather owned before the first robbery."

"The books will show it," she said simply.

"They will?" commented Kennedy. "Then if you will explain to me briefly just the system you used in keeping account of it, perhaps I need not trouble you any more."

"I'll go down there with you," she answered bravely. "I'm better to-day, anyhow, I think."

She had risen, but it was evident that she was not as strong as she wanted us to think.

"The least I can do is to make it as easy as possible by going in a car," remarked Kennedy, following her into the hall where there was a telephone.

The hallway was perfectly dark, yet as she preceded us I could see that the diamond pin which held her collar in the back sparkled as if a lighted candle had been brought near it. I had noticed in the parlor that she wore a handsome tortoiseshell comb set with what I thought were other brilliants, but when I looked I saw now that there was not the same sparkle to the comb which held her dark hair in a soft mass. I noticed these little things at the time, not because I thought they had any importance, but merely by chance, wondering at the sparkle of the one diamond which had caught my eye.

"What do you make of her?" I asked as Kennedy finished telephoning.

"A very charming and capable girl," he answered noncommittally.

"Did you notice how that diamond in her neck sparkled?" I asked quickly.

He nodded. Evidently it had attracted his attention, too.

"What makes it?" I pursued.

"Well, you know radium rays will make a diamond fluoresce in the dark."

"Yes," I objected, "but how about those in the comb?"

"Paste, probably," he answered tersely, as we heard her foot on the landing. "The rays won't affect paste."

It was indeed a shame to take advantage of Miss Wallace's loyalty to Denison, but she was so game about it that I knew only the utmost necessity on Kennedy's part would have prompted him to do it. She had a key to the office so that it was not necessary to wait for Denison, if indeed we could have found him.

Together she and Kennedy went over the records. It seemed that there were in the safe twenty-five platinum tubes of one hundred milligrams each, and that there had been twelve of the same amount at Pittsburgh. Little as it seemed in weight it represented a fabulous fortune.

"You have not the combination?" inquired Kennedy.

"No. Only Mr. Denison has that. What are you going to do to protect the safe to-night?" she asked.

"Nothing especially," evaded Kennedy.

"Nothing?" she repeated in amazement.

"I have another plan," he said, watching her intently. "Miss Wallace, it was too much to ask you to come down here. You are ill."

She was indeed quite pale, as if the excitement had been an overexertion.

"No, indeed," she persisted. Then, feeling her own weakness, she moved toward the door of Denison's office where there was a leather couch. "Let me rest here a moment. I do feel queer. I—"

She would have fallen if he had not sprung forward and caught her as she sank to the floor, overcome by the exertion.

Together we carried her in to the couch, and as we did so the comb from her hair clattered to the floor.

Craig threw open the window, and bathed her face with water until there was a faint flutter of the eyelids.

"Walter," he said, as she began to revive, "I leave her to you. Keep her quiet for a few moments. She has unintentionally given me just the opportunity I want."

While she was yet hovering between consciousness and unconsciousness on the couch, he had unwrapped the package which he had brought with him. For a moment he held the comb which she had dropped near the radioscope. With a low exclamation of surprise he shoved it into his pocket.

Then from the package he drew a heavy piece of apparatus which looked as if it might be the motor part of an electric fan, only in place of the fan he fitted a long, slim, vicious-looking steel bit. A flexible wire attached the thing to the electric light circuit and I knew that it was an electric drill. With his coat off he tugged at the little radium safe until he had moved it out, then dropped on his knees behind it and switched the current on in the electric drill.

It was a tedious process to drill through the steel of the outer casing of the safe and it was getting late. I shut the door to the office so that Miss Wallace could not see.

At last by the cessation of the low hum of the boring, I knew that he had struck the inner lead lining. Quietly I opened the door and stepped out. He was injecting something from an hermetically sealed lead tube into the opening he had made and allowing it to run between the two linings of lead and steel. Then using the tube itself he sealed the opening he had made and dabbed a little black over it.

Quickly he shoved the safe back, then around it concealed several small coils with wires also concealed and leading out through a window to a court.

"We'll catch the fellow this time," he remarked as he worked. "If you ever have any idea, Walter, of going into the burglary business, it would be well to ascertain if the safes have any of these little selenium cells as suggested by my friend, Mr. Hammer, the inventor. For by them an alarm can be given miles away the moment an intruder's bull's-eye falls on a hidden cell sensitive to light."

While I was delegated to take Miss Wallace home, Kennedy made arrangements with a small shopkeeper on the ground floor of a building that backed up on the court for the use of his back room that night, and had already set up a bell actuated by a system of relays which the weak current from the selenium cells could operate.

It was not until nearly midnight that he was ready to leave the laboratory again, where he had been busily engaged in studying the tortoiseshell comb which Miss Wallace in her weakness had forgotten.

The little shopkeeper let us in sleepily and Kennedy deposited a large round package on a chair in the back of the shop, as well as a long piece of rubber tubing. Nothing had happened so far.

As we waited the shopkeeper, now wide awake and not at all unconvinced that we were bent on some criminal operation, hung around. Kennedy did not seem to care. He drew from his pocket a little shiny brass instrument in a lead case, which looked like an abbreviated microscope.

"Look through it," he said, handing it to me.

I looked and could see thousands of minute sparks.

"What is it?" I asked.

"A spinthariscope. In that it is possible to watch the bombardment of the countless little corpuscles thrown off by radium, as they strike on the zinc blende crystal which forms the base. When radium was originally discovered, the interest was merely in its curious properties, its power to emit invisible rays which penetrated solid substances and rendered things fluorescent, of expending energy without apparent loss.

"Then came the discovery," he went on, "of its curative powers. But the first results were not convincing. Still, now that we know the reasons why radium may be dangerous and how to protect ourselves against them we know we possess one of the most wonderful of curative agencies."

I was thinking rather of the dangers than of the beneficence of radium just now, but Kennedy continued.

"It has cured many malignant growths that seemed hopeless, brought back destroyed cells, exercised good effects in diseases of the liver and intestines and even the baffling diseases of the arteries. The reason why harm, at first, as well as good came, is now understood. Radium emits, as I told you before, three kinds of rays, the alpha, beta, and gamma rays, each with different properties. The emanation is another matter. It does not concern us in this case, as you will see."

Fascinated as I was by the mystery of the case, I began to see that he was gradually arriving at an explanation which had baffled everyone else.

"Now, the alpha rays are the shortest," he launched forth, "in length let us say one inch. They exert a very destructive effect on healthy tissue. That is the cause of injury. They are stopped by glass, aluminum and other metals, and are really particles charged with positive electricity. The beta rays come next, say, about an inch and a half. They stimulate cell growth. Therefore they are dangerous in cancer, though good in other ways. They can be stopped by lead, and are really particles charged with negative electricity. The gamma rays are the longest, perhaps three inches long, and it is these rays which effect cures, for they check the abnormal and stimulate the normal cells. They penetrate lead. Lead seems to filter them out from the other rays. And at three inches the other rays don't reach, anyhow. The gamma rays are not charged with electricity at all, apparently."

He had brought a little magnet near the spinthariscope. I looked into it.

"A magnet," he explained, "shows the difference between the alpha, beta, and gamma rays. You see those weak and wobbly rays that seem to fall to one side? Those are the alpha rays. They have a strong action, though, on tissues and cells. Those falling in the other direction are the beta rays. The gamma rays seem to flow straight."

"Then it is the alpha rays with which we are concerned mostly now?" I queried, looking up.

"Exactly. That is why, when radium is unprotected or insufficiently protected and comes too near, it is destructive of healthy cells, produces burns, sores, which are most difficult to heal. It is with the explanation of such sores that we must deal."

It was growing late. We had waited patiently now for some time. Kennedy had evidently reserved this explanation, knowing we should have to wait. Still nothing happened.

Added to the mystery of the violet-colored glass plate was now that of the luminescent diamond. I was about to ask Kennedy point-blank what he thought of them, when suddenly the little bell before us began to buzz feebly under the influence of a current.

I gave a start. The faithful little selenium cell burglar alarm had done the trick. I knew that selenium was a good conductor of electricity in the light, poor in the dark. Some one had, therefore, flashed a light on one of the cells in the Corporation office. It was the moment for which Kennedy had prepared.

Seizing the round package and the tubing, he dashed out on the street and around the corner. He tried the door opening into the Radium Corporation hallway. It was closed, but unlocked. As it yielded and we stumbled in, up the old worn wooden stairs of the building, I knew that there must be some one there.

A terrific, penetrating, almost stunning odor seemed to permeate the air even in the hall.

Kennedy paused at the door of the office, tried it, found it unlocked, but did not open it.

"That smell is ethyldichloracetate," he explained. "That was what I injected into the air cushion of that safe between the two linings. I suppose my man here used an electric drill. He might have used thermit or an oxyacetylene blowpipe for all I would care. These fumes would discourage a cracksman from 'soup' to nuts," he laughed, thoroughly pleased at the protection modern science had enabled him to devise.

As we stood an instant by the door, I realized what had happened. We had captured our man. He was asphyxiated!

Yet how were we to get to him? Would Craig leave him in there, perhaps to die? To go in ourselves meant to share his fate, whatever might be the effect of the drug.

Kennedy had torn the wrapping off the package. From it he drew a huge globe with bulging windows of glass in the front and several curious arrangements on it at other points. To it he fitted the rubber tubing and a little pump. Then he placed the globe over his head, like a diver's helmet, and fastened some air-tight rubber arrangement about his neck and shoulders.

"Pump, Walter!" he shouted. "This is an oxygen helmet such as is used in entering mines filled with deadly gases."

Without another word he was gone into the blackness of the noxious stifle which filled the Radium Corporation office since the cracksman had struck the unexpected pocket of rapidly evaporating stuff.

I pumped furiously.

Inside I could hear him blundering around. What was he doing?

He was coming back slowly. Was he, too, overcome?

As he emerged into the darkness of the hallway where I myself was almost sickened, I saw that he was dragging with him a limp form.

A rush of outside air from the street door seemed to clear things a little. Kennedy tore off the oxygen helmet and dropped down on his knees beside the figure, working its arms in the most approved manner of resuscitation.

"I think we can do it without calling on the pulmotor," he panted.
"Walter, the fumes have cleared away enough now in the outside office.
Open a window—and keep that street door open, too."

I did so, found the switch and turned on the lights.

It was Denison himself!

For many minutes Kennedy worked over him. I bent down, loosened his collar and shirt, and looked eagerly at his chest for the tell-tale marks of the radium which I felt sure must be there. There was not even a discoloration.

Not a word was said, as Kennedy brought the stupefied little man around.

Denison, pale, shaken, was leaning back now in a big office chair, gasping and holding his head.

Kennedy, before him, reached down into his pocket and handed him the spinthariscope.

"You see that?" he demanded.

Denison looked through the eyepiece.

"Wh—where did you get so much of it?" he asked, a queer look on his face.

"I got that bit of radium from the base of the collar button of Hartley Haughton," replied Kennedy quietly, "a collar button which some one intimate with him had substituted for his own, bringing that deadly radium with only the minutest protection of a thin strip of metal close to the back of his neck, near the spinal cord and the medulla oblongata which controls blood pressure. That collar button was worse than the poisoned rings of the Borgias. And there is more radium in the pretty gift of a tortoiseshell comb with its paste diamonds which Miss Wallace wore in her hair. Only a fraction of an inch, not enough to cut off the deadly alpha rays, protected the wearers of those articles."

He paused a moment, while surging through my mind came one after another the explanations of the hitherto inexplicable. Denison seemed almost to cringe in the chair, weak already from the fumes.

"Besides," went on Kennedy remorselessly, "when I went in there to drag you out, I saw the safe open. I looked. There was nothing in those pretty platinum tubes, as I suspected. European trust—bah! All the cheap devices of a faker with a confederate in London to send a cablegram—and another in New York to send a threatening letter."

Kennedy extended an accusing forefinger at the man cowering before him.

"This is nothing but a get-rich-quick scheme, Denison. There never was a milligram of radium in the Poor Little Rich Valley, not a milligram here in all the carefully kept reports of Miss Wallace—except what was bought outside by the Corporation with the money it collected from its dupes. Haughton has been fleeced. Miss Wallace, blinded by her loyalty to you—you will always find such a faithful girl in such schemes as yours—has been fooled.

"And how did you repay it? What was cleverer, you said to yourself, than to seem to be robbed of what you never had, to blame it on a bitter rival who never existed? Then to make assurance doubly sure, you planned to disable, perhaps get rid of the come-on whom you had trimmed, and the faithful girl whose eyes you had blinded to your gigantic swindle.

"Denison," concluded Kennedy, as the man drew back, his very face convicting him, "Denison, you are the radium robber—robber in another sense!"