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As Kennedy walked through the corridor of the building, he paused and bent down, as though examining the wall. I looked, too. There was a crack in the concrete, in the side wall toward the Creighton laboratory.

“Do you suppose vibration caused it?” I asked, remembering his watch crystal test.

Craig shook his head. “The vibrations in a building can be shown by a watch glass full of water. You saw the surface of the liquid with its minute waves. There’s vibration, all right, but that is not the cause of such cracks as these.”

He stood for a moment regarding the crack attentively. On the floor on which we were was the Consolidated Bank itself. Beneath us were the Consolidated Safety Deposit vaults.

“What did cause them, then?” I asked, mystified.

“Apparently escaping currents of electricity are causing electrolysis of the Bank Building,” he replied, his face wrinkled in thought.

“Electrolysis?” I repeated mechanically.

“Yes. I suppose you know how stray or vagrant currents affect steel and concrete?”

I shook my head in the negative.

“Well,” he explained as we stood there, “I believe that in one government test at least it was shown that when an electric current of high voltage passes from steel to concrete, the latter is cracked and broken. Often a mechanical pressure as great as four or five thousand pounds a square inch is exerted and there is rapid destruction due to the heating effect of the current.”

I expressed my surprise at what he had discovered. “The danger is easily overestimated,” he hastened to add. “But in this case I think it is real, though probably it is a special and extreme condition. Still it is special and extreme conditions which we are in the habit of encountering in our cases, Walter. That is what we must be looking out for. In this instance the destruction due to electrolysis is most likely caused by the oxidation of the iron anode. The oxides which are formed are twice as great in volume as the iron was originally and the resulting pressure is what causes the concrete to break. I think we shall find that this condition will bear strict watching.”

For a moment Kennedy stopped at the little office of the superintendent of the building, in the rear.

“I was just wondering whether you had noticed those cracks in the walls down the corridor,” remarked Kennedy after a brief introduction.

The superintendent looked at him suspiciously. Evidently he feared we had some ulterior motive, perhaps represented some rival building and might try to scare away his tenants.

“Oh, that’s nothing,” he said confidently. “Just the building settling a bit–easily fixed.”

“The safety vault company haven’t complained?” persisted Kennedy, determined to get something out of the agent.

“No indeed,” he returned confidently. “I guess they’ve got troubles of their own–real ones.”

“How’s that?” asked Craig, falling in with the man’s evident desire to change the subject.

“Why, I believe their alarm system’s out of order,” he replied. “Some of the fine wires in it burnt out, I think. Defective wiring, I guess. Oh, they’ve had it patched up, changed about a little,–it’s all right now, they say. But they’ve had a deuce of a time with the alarm ringing at all sorts of hours, and not a trace of trouble.”

I looked quickly at Craig. Though the superintendent thought he had been very clever in changing the topic of conversation, he had unwittingly furnished us with another clew. I could not ask Craig before him and I forgot to do so later, but, to me at least, it seemed as if this might be due to induction from the stray currents.

“No one here seems to have suspected the Creighton motor, anyhow,” commented Craig to me, as we thanked the superintendent and walked across to the elevators.

We rode up to Tresham’s office, which was on the third floor, on the side of the building toward Creighton’s laboratory. In fact one of the windows opened almost on the roof of the brick building next door.