"Why, you might cook in it.

I say, bo'sun, look ye here; why if it aren't just like the stuff as my old mother used to scrape out of the tea-kettle at home." Small stooped and broke off a scrap of the deposit, and examined it, holding it out afterwards to Mark.
"Yes," said the major, who examined it in turn, after Mark had taken it to him, "the man is quite right.

It is a limy deposit from the boiling water, similar to what is found in kettles and boilers.

Shows that the water is very hard, eh, captain ?" "Yes, I suppose that's it," replied Captain Strong.

"But all this is very interesting for travellers, and does not concern us.