These different works lasted a week.  They were finished before the transformation of the sulphuret into sulphate of iron had been accomplished.  During the following days the settlers had time to construct a furnace of bricks of a particular arrangement, to serve for the distillation of the sulphate or iron when it had been obtained.  All this was finished about the 18th of May, nearly at the time when the chemical transformation terminated.  Gideon Spilett, Herbert, Neb, and Pencroft, skillfully directed by the engineer, had become most clever workmen.  Before all masters, necessity is the one most listened to, and who teaches the best.

When the heap of pyrites had been entirely reduced by fire, the result of the operation, consisting of sulphate of iron, sulphate of alumina, flint, remains of coal, and cinders was placed in a basinful of water.  They stirred this mixture, let it settle, then decanted it, and obtained a clear liquid containing in solution sulphate of iron and sulphate of alumina, the other matters remaining solid, since they are insoluble.  Lastly, this liquid being partly evaporated, crystals of sulphate of iron were deposited, and the not evaporated liquid, which contained the sulphate of alumina, was thrown away.

Cyrus Harding had now at his disposal a large quantity of these sulphate of iron crystals, from which the sulphuric acid had to be extracted.  The making of sulphuric acid is a very expensive manufacture.  Considerable works are necessary—­a special set of tools, an apparatus of platina, leaden chambers, unassailable by the acid, and in which the transformation is performed, etc.  The engineer had none of these at his disposal, but he knew that, in Bohemia especially, sulphuric acid is manufactured by very simple means, which have also the advantage of producing it to a superior degree of concentration.  It is thus that the acid known under the name of Nordhausen acid is made.

To obtain sulphuric acid, Cyrus Harding had only one operation to make, to calcine the sulphate of iron crystals in a closed vase, so that the sulphuric acid should distil in vapor, which vapor, by condensation, would produce the acid.

The crystals were placed in pots, and the heat from the furnace would distil the sulphuric acid.  The operation was successfully completed, and on the 20th of May, twelve days after commencing it, the engineer was the possessor of the agent which later he hoped to use in so many different ways.

Now, why did he wish for this agent?  Simply to produce azotic acid; and that was easy, since saltpeter, attacked by sulphuric acid, gives azotic, or nitric, acid by distillation.

But, after all, how was he going to employ this azotic acid?  His companions were still ignorant of this, for he had not informed them of the result at which he aimed.

However, the engineer had nearly accomplished his purpose, and by a last operation he would procure the substance which had given so much trouble.