“But,” resumed Kennedy, “the bends are still hitting the men, and there is no telling when a fire or a blow-out may occur in any of the eight headings that are now being pushed under the river.  Quite often the work has been delayed and the tunnel partly or wholly flooded.  Now, you know the theory of the bends.  It is that air—­mostly the nitrogen in the air—­is absorbed by the blood under the pressure.  In coming out of the ‘air’ if the nitrogen is not all eliminated, it stays in the blood and, as the pressure is reduced, it expands.  It is just as if you take a bottle of charged water and pull the cork suddenly.  The gas rises in big bubbles.  Cork it again and the gas bubbles cease to rise and finally disappear.  If you make a pin-hole in the cork the gas will escape slowly, without a bubble.  You must decompress the human body slowly, by stages, to let the super-saturated blood give up its nitrogen to the lungs, which can eliminate it.  Otherwise these bubbles catch in the veins, and the result is severe pains, paralysis, and even death.  Gentlemen, I see that I am just wasting time telling you this, for you know it all well.  But consider.”

Kennedy placed an empty corked flask on the table.  The others regarded it curiously, but I recalled having seen it in the tunnel.

“In this bottle,” explained Kennedy, “I collected some of the air from the tunnel when I was down there this morning.  I have since analysed it.  The quantity of carbon dioxide is approximately what it should be—­not high enough of itself to cause trouble.  But,” he spoke slowly to emphasise his words, “I found something else in that air beside carbon dioxide.”

“Nitrogen?” broke in Orton quickly, leaning forward.

“Of course; it is a constituent of air.  But that is not what I mean.”

“Then, for Heaven’s sake, what did you find?” asked Orton.

“I found in this air,” replied Kennedy, “a very peculiar mixture—­ an explosive mixture.”

“An explosive mixture?” echoed Orton.

“Yes, Jack, the blow-outs that you have had at the end of the tunnel were not blow-outs at all, properly speaking.  They were explosions.”

We sat aghast at this revelation.

“And, furthermore,” added Kennedy, “I should, if I were you, call back all the men from the tunnel until the cause for the presence of this explosive mixture is discovered and remedied.”

Orton reached mechanically for the telephone to give the order, but Taylor laid his hand on his arm.  “One moment, Orton,” he said.  “Let’s hear Professor Kennedy out.  He may be mistaken, and there is no use frightening the men, until we are certain.”

“Shelton,” asked Kennedy, “what sort of flash oil is used to lubricate the machinery?”

“It is three-hundred-and-sixty-degree Fahrenheit flash test,” he answered tersely.

“And are the pipes leading air down into the tunnel perfectly straight?”