They dug the compacted oakum from the seams with their knives, and by iron rings in each corner, now eaten with rust to almost the thinness of wire, they lifted the hatch.  Below was a filthy-looking layer of whitish substance, protruding from which were charred, half-burned staves.  First they repeated the experiment with the smouldering rag, and finding that it burned, as before, they descended.  The whitish substance was hard enough to bear their weight, and they looked around.  Overhead, hung to the under side of the deck and extending the length of the hold, were wooden tanks, charred, and in some places burned through.

“She must have been built for a passenger or troop ship,” said Boston.  “Those tanks would water a regiment.”

“Boston,” answered the doctor, irrelevantly, “will you climb up and bring down an oar from the boat?  Carry it down—­don’t throw it, my boy.”  Boston obliged him, and the doctor, picking his way forward, then aft, struck each tank with the oar.  “Empty—­all of them,” he said.

He dug out with his knife a piece of the whitish substance under foot, and examined it closely in the light from the hatch.

“Boston,” he said, impressively, “this ship was loaded with lime, tallow, and acids—­acids above, lime and tallow down here.  This stuff is neither; it is lime-soap.  And, moreover, it had not been touched by acids.”  The doctor’s ruddy face was ashen.

“Well?” asked Boston.

“Lime soap is formed by the cauticizing action of lime on tallow in the presence of water and heat.  It is easy to understand this fire.  One of those tanks leaked and dribbled down on the cargo, attacking the lime—­which was stowed underneath, as all these staves we see on top are from tallow-kids.  The heat generated by the slaking lime set fire to the barrels in contact, which in turn set fire to others, and they burned until the air was exhausted, and then went out.  See, they are but partly consumed.  There was intense heat in this hold, and expansion of the water in all the tanks.  Are tanks at sea filled to the top?”

“Chock full, and a cap screwed down on the upper end of the pipes.”

“As I thought.  The expanding water burst every tank in the hold, and the cargo was deluged with water, which attacked every lime barrel in the bottom layer, at least.  Result—­the bursting of those barrels from the ebullition of slaking lime, the melting of the tallow—­which could not burn long in the closed-up-space—­and the mixing of it in the interstices of the lime barrels with water and lime—­a boiling hot mess.  What happens under such conditions?”

“Give it up,” said Boston, laconically.

“Lime soap is formed, which rises, and the water beneath is in time all taken up by the lime.”

“But what of it?” interrupted the other.

“Wait.  I see that this hold and the ’tween-deck are lined with wood.  Is that customary in iron ships?”