“I mean,” said the doctor, emphatically, “that the first thing the acids—­mixed in the ’tween-deck to just about the right proportions, mind you—­would attack, on oozing through the skin, would be this glycerine; and the certain product of this union under intense cold—­this hull was frozen in the ice, remember—­would be nitro-glycerine; and, as the yield of the explosive is two hundred and twenty per cent. of the glycerine, we can be morally sure that in the bottom of this hold, each minute globule of it held firmly in a hard matrix of sulphate or nitrate of calcium—­which would be formed next when the acids met the hydrates and carbonates of lime—­is over one hundred and thirty tons of nitro-glycerine, all the more explosive from not being washed of free acids.  Come up on deck.  I’ll show you something else.”

Limp and nerveless, Boston followed the doctor.  This question was beyond his seamanship.

The doctor brought the yellow substance—­now well dried.  “I found plenty of this in the ’tween-deck,” he said; “and I should judge they used it to pack between the carboy boxes.  It was once cotton-batting.  It is now, since I have washed it, a very good sample of gun-cotton.  Get me a hammer—­crowbar—­something hard.”

Boston brought a marline-spike from the locker, and the doctor, tearing off a small piece of the substance and placing it on the iron barrel of a gipsy-winch, gave it a hard blow with the marline spike, which was nearly torn from his hand by the explosion that followed.

“We have in the ’tween-deck,” said the doctor, as he turned, “about twice as many pounds of this stuff as they used to pack the carboys with; and, like the nitro-glycerine, is the more easily exploded from the impurities and free acids.  I washed this for safe handling.  Boston, we are adrift on a floating bomb that would pulverize the rock of Gibraltar!”

“But, doctor,” asked Boston, as he leaned against the rail for support, “wouldn’t there be evolution of heat from the action of the acids on the lime—­enough to explode the nitro-glycerine just formed?”

“The best proof that it did not explode is the fact that this hull still floats.  The action was too slow, and it was very cold down there.  But I can’t yet account for the acids left in the bilges.  What have they been doing all these fifty years?”

Boston found a sounding-rod in the locker, which he scraped bright with his knife, then, unlaying a strand of the rope for a line, sounded the pump-well.  The rod came up dry, but with a slight discoloration on the lower end, which Boston showed to the doctor.

“The acids have expended themselves on the iron frames and plates.  How thick are they?”

“Plates, about five-eighths of an inch; frames, like railroad iron.”

“This hull is a shell!  We won’t get much salvage.  Get up some kind of distress signal, Boston.”  Somehow the doctor was now the master-spirit.