n at once, boatswain," said
Curtis.

"But if it is to take us a month, captain, perhaps by that time the ship
may be knocked to atoms. Couldn't we manage to blow up the rock? we have
got some powder on board."

"Not enough for that;" said the boatswain.

"You have something better than powder," said Falsten.

"What's that?" asked the captain.

"Picrate of potash," was the reply.

And so the explosive substance with which poor Ruby had so grievously
imperilled the vessel was now to serve her in good stead, and I now saw
what a lucky thing it was that the case had been deposited safely on the
reef, instead of being thrown into the sea.

Picric acid is a crystalline bitter product extracted from coal-tar, and
forming, in combination with potash, a yellow salt known as picrate of
potash. The explosive power of this substance is inferior to that
of gun-cotton or of dynamite, but far greater than that of ordinary
gunpowder; one grain of picric powder producing an effect equal to that
of thirteen grains of common powder. Picrate is easily ignited by
any sharp or violent shock, and some gun-priming which we had in our
possession would answer the purpose of setting it alight.

The sailors went off at once for their pikes, and Dowlas and his
assistants, under the direction of Falsten, who, as an engineer,
understood such matters, proceeded to hollow out a mine wherein to
deposit the powder. At first we hoped that everything would be ready for
the blasting to take place on the following morning, but when daylight
appeared we found that the men, although they had laboured with a will,
had only been able to work for an hour at low water and that four tides
must ebb before the mine had been sunk to the required depth.

Not until eight o'clock on the morning of the 23rd was the work
complete. The hole was bored obliquely in the rock, and was large enough
to contain about ten pounds of explosive matter. Just as the picrate was
being introduced into the aperture, Falsten interposed:--

"Stop," he said, "I think it will be best to mix the picrate with common
powder, as that will allow us to fire the mine with a match instead of
the gun-priming which would be necessary to produce a shock. Besides,
it is an understood thing that the addition of gunpowder renders picrate
far more effective in blasting such rocks as this, as then the violence
of the picrate prepares the way for the powder which, slower in its
action, will complet