Taking some azotic acid, he mixed it with glycerine, which had been previously concentrated by evaporation, subjected to the water-bath, and he obtained, without even employing a refrigerant mixture, several pints of an oily yellow mixture.

This last operation Cyrus Harding had made alone, in a retired place, at a distance from the Chimneys, for he feared the danger of an explosion, and when he showed a bottle of this liquid to his friends, he contented himself with saying,—­

“Here is nitro-glycerine!”

It was really this terrible production, of which the explosive power is perhaps tenfold that of ordinary powder, and which has already caused so many accidents.  However, since a way has been found to transform it into dynamite, that is to say, to mix with it some solid substance, clay or sugar, porous enough to hold it, the dangerous liquid has been used with some security.  But dynamite was not yet known at the time when the settlers worked on Lincoln Island.

“And is it that liquid that is going to blow up our rocks?” said Pencroft incredulously.

“Yes, my friend,” replied the engineer, “and this nitro-glycerine will produce so much the more effect, as the granite is extremely hard, and will oppose a greater resistance to the explosion.”

“And when shall we see this, captain?”

“To-morrow, as soon as we have dug a hole for the mine, replied the engineer.”

The next day, the 21st of May, at daybreak, the miners went to the point which formed the eastern shore of Lake Grant, and was only five hundred feet from the coast.  At this place, the plateau inclined downwards from the waters, which were only restrained by their granite case.  Therefore, if this case was broken, the water would escape by the opening and form a stream, which, flowing over the inclined surface of the plateau, would rush on to the beach.  Consequently, the level of the lake would be greatly lowered, and the opening where the water escaped would be exposed, which was their final aim.

Under the engineer’s directions, Pencroft, armed with a pickaxe, which he handled skillfully and vigorously, attacked the granite.  The hole was made on the point of the shore, slanting, so that it should meet a much lower level than that of the water of the lake.  In this way the explosive force, by scattering the rock, would open a large place for the water to rush out.

The work took some time, for the engineer, wishing to produce a great effect, intended to devote not less than seven quarts of nitro-glycerine to the operation.  But Pencroft, relieved by Neb, did so well, that towards four o’clock in the evening, the mine was finished.

Now the question of setting fire to the explosive substance was raised.  Generally, nitro-glycerine is ignited by caps of fulminate, which in bursting cause the explosion.  A shock is therefore needed to produce the explosion, for, simply lighted, this substance would burn without exploding.