the disc, for experimental purposes, was being placed in a cask floating in the water. On the fuse being fired, this cask was blown “into matchwood”—a wreck so complete that the most ignorant spectator could not fail to understand what would have been the fate of a boat and its crew in similar circumstances.

“How very awful!” said my mother. “Pray, Mr Firebrand, what is gun-worsted—I mean cotton.”

The young lieutenant smiled rather broadly as he explained, in a glib and slightly sing-song tone, which savoured of the Woolwich Military Academy, that, “gun-cotton is the name given to the explosive substance produced by the action of nitric acid mixed with sulphuric acid, on cotton fibre.” He was going to add, “It contains carbon, hydrogen, nitrogen, and oxygen, corresponding to—” when my mother stopped him.

“Dear me, Mr Firebrand, is a popular explanation impossible?”

“Not impossible, madam, but rather difficult. Let me see. Gun-cotton is a chemical compound of the elements which I have just named—a chemical compound, you will observe, not a mechanical mixture, like gunpowder. Hence it explodes more rapidly than the latter, and its power is from three to six times greater.”

My mother looked perplexed. “What is the difference,” she asked, “between a chemical compound and a mechanical mixture?”

Firebrand now in his turn looked perplexed. “Why, madam,” he exclaimed, in modulated desperation, “the ultimate molecules of a mixture are only placed beside each other, so that an atom of gunpowder may be saltpetre, charcoal, or sulphur, dependent on its fellow-atoms for power to act; whereas a chemical compound is such a perfect union of substances, that each ultimate molecule is complete in its definite proportions of the four elements, and therefore an independent little atom.”

“Now, the next experiment,” continued Firebrand, glad to have an opportunity of changing the subject, “is meant to illustrate our method of countermining. You must know that our enemies may sometimes sink torpedoes at the entrance of their harbours, to prevent our ships of war entering. Such torpedoes consist usually of casks or cases of explosives, which are fired either by electric wires, like the telegraph, when ships are seen to be passing over them, or by contact. That is to say, an enemy’s ship entering a harbour runs against something which sets something else in motion, which explodes the torpedo and blows it and the ship into what natives of the Green Isle call smithereens. This is 

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