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.