We notice this subject more particularly, since, as in the different fire-works, nitre and inflammable bodies are used in different proportions, the result is always affected by the same laws of chemical decomposition; for the same substances, placed under similar circumstances of proportion, mixture, &c. afford the like results. If carbon alone be employed, carbonic acid gas is the result; if oil, tallow, rosin, or turpentine be used, we have then, as we had occasion to remark, water, as well as carbonic acid, by reason of the union of the hydrogen, which forms one of their constituent parts, with a part of the oxygen of the nitric acid.
Again, in a composition of mealed powder, rosin and sulphur, with or without the addition of saw dust, we infer, from the composition of the ingredients and the chemical action which subsequently takes place, that the products of combustion would be carbonic acid gas, sulphurous acid gas, water, sulphuretted hydrogen, and probably azotic, and nitric oxide gases. If the filings of steel, brass, zinc, or copper, enter into the composition, besides the products above-mentioned, there would be either an oxide of iron, an oxide of zinc, or, an oxide of copper, according as one or other of these metals are employed.
Copper, in fire-works, has the effect of communicating a green colour to the flame. M. Homberg, (_Collection Acad._) observes, that the green colour in such cases is owing to the _dissolution_ of the metal, which in fact is nothing more than the _effect_ of its oxidizement.
The various compositions for brilliant fire, as the Chinese fire, owe their peculiar character to pulverised cast iron, and commonly to steel and iron filings. Now the effects in these cases are the same; for the same oxidizement ensues, more or less rapidly, which in fact distinguishes the kinds of brilliant fire. That of the Chinese is the most perfect, and next is the composition made with steel filings. It will be seen, however, that compositions generally are governed, in their respective appearances when inflamed, by the purity, as well as the proportion of other substances, which enter into them; and hence much of their effect depends on collateral circumstances, which we purpose to consider when we treat of the compositions individually.