In military fire-works, tow and hemp are much used, and principally for the preparation of incendiary works. Both tow and hemp are employed in forming match. Although old rope, &c. are used for immersion in the tourteaux, carcass, or fire-stone composition, which is readily imbibed, if the rope is untwisted and beaten; yet tow or hemp is a better material, and receives more of the composition. The manner of using it may be seen by referring to the composition for fire-stone. For very nice purposes, the tow or hemp should be well dressed. Flax is, therefore, to be preferred in such cases. _Sec. LVI. Of Blue Vitriol._ Different preparations of copper are used in fire-works, to communicate colour to the flame; and besides copper filings, brass filings, verdigris, and the oxides of copper, the sulphate of copper, or blue vitriol, has been employed. We may observe here, that there are three sub-species of this salt; the bisulphate, sulphate, and sub-sulphate, the first properly speaking being the blue vitriol of commerce. The sulphate, although recommended in some of the old formulæ for coloured fire, is not, however, preferable to some other preparations of copper. The use and application of copper, and its preparations, will be seen in the article on coloured fire. When sulphate of copper is heated, it is converted into a bluish-white powder. If the heat be increased, the acid is expelled, and the black oxide of copper remains. Before it is used, it is exposed to heat to expel the water of crystallization. It ought to be in the state of impalpable powder. It is composed of 33 acid, 32 oxide, and 35 water. It is decomposed by the alkalies and earths, the alkaline carbonates, borates, and phosphates, and several metallic salts. The oxide may be obtained very readily from this salt, for the purpose of fire-works, by dissolving it in water, and adding a solution of caustic potassa; collecting the precipitate, and drying it in a moderate heat. This will expel the water that may be contained in it; as metallic precipitates, made in this way, are more or less in the state of hydrates. When metallic copper is required, it may be obtained in fine powder, and very expeditiously, by immersing a plate of iron in a solution of any of the salts of copper, as the sulphate. It will precipitate on the iron, and gradually fall to the bottom of the vessel. This metallic copper will be found to be much more impalpable than the filings, however fine, and, for that reason, may be mixed more accurately with different substances. Copper burns with a beautiful green flame, and deposites a loose greenish-gray oxide. The ammonia-oxalate of copper, of which there are three sub-species, burns with flame. _Sec. LVII. Of Nitrate of Copper._