The second change, with respect to the appearance of the flame, the formation of stars, serpents, rain, &c. terms used in the art, is owing either to new chemical changes which the substances undergo, or to the decomposition of the products themselves. These effects, it is obvious, must be governed by the circumstances, under which the mixtures are made. Saltpetre, for instance, is the basis of fire-works, whether used in a separate state, or employed in mixture with charcoal and sulphur, as in gun-powder; and, from its composition, is adapted to all the purposes of the art, because it yields its oxygen very readily to all inflammable bodies. In consequence of the decomposition, it undergoes at an elevated temperature, when brought in contact with charcoal, sulphur, &c. and various substances which contain carbon, as pitch, rosin, turpentine, tallow, copal, and amber, combustion results, and, according to circumstances, is more or less rapid, and the flame also more or less brilliant.
When charcoal, in the state of ignition, is brought in contact with nitre, a deflagration takes place, because, at the temperature of ignition, it has the property of decomposing the nitric acid of the nitre; and as this process unites the carbon with the oxygen, in the proportion necessary to constitute carbonic acid, this acid is accordingly produced. When, therefore, we inflame a mixture of nitre, charcoal, and sulphur, or gun-powder, the whole or greater part disappears; and if we were to collect in a pneumatic apparatus, the products of the combustion, it would be found, that they are nearly altogether gaseous, and composed, as we shall speak hereafter, of sundry elastic aëriform fluids. This decomposition, the immediate effect of the charcoal on the nitric acid of the nitre, is the same as in the preceding instance, for carbonic acid gas is formed in both cases. We have then another instance of combustion, where a number of substances are concerned, and therefore, the products must be numerous.
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.
That the light of certain burning bodies may be increased, is evident from these facts; and experiment has shown, that the intensity of the light of burning sulphur, hydrogen, carbonic oxide, &c. is increased by throwing into them, zinc, or its oxide, iron, and other metals, or by placing in them very fine amianthus or metallic gauze. Protochloride of copper burns with a dense red light, tinged with green and blue towards the edges. If the hydrogen of the oil acts in separating the chlorine from the copper, and the reduced copper is ignited by the charcoal, this appearance must necessarily ensue.
When solid matter is the product of combustion, as in the burning of phosphorus, zinc, iron, &c. the flame is remarked to be more intense. Flame may be modified under other circumstances, as we will have occasion to mention hereafter. When, for instance, a wire-gauze safety-lamp is made to burn in a very explosive mixture of coal gas and air, the light is very feeble and of a pale colour; but when a current of coal gas is burnt in atmospheric air, the combustion is rapid and the flame brilliant.
Dr. Ure thinks it probable, (Dictionary of Chemistry, article combustion,) that, when the colour of the flame is changed by the introduction of incombustible compounds, the effect depends on the production, and subsequent ignition or combustion of inflammable matter from them. Thus he infers, that the rose-coloured light given to flame by the compounds of strontium and calcium, and the yellow colour given by those of barium, and the green by those of boron, may depend upon a temporary production of these bases, by the inflammable matter of the flame. It is inferred also, as a probable conclusion, that the heat of flames may be actually diminished by increasing their light, (at least the heat communicable to other matter), and vice versa; because, in the most intense heat, as in the compound blow pipe, or in Newman's blow pipe apparatus, in which a mixture of oxygen and hydrogen gases is compressed, the flame, although hardly visible in bright day light, instantly fuses the most refractory bodies; but the light of solid bodies ignited in it, is so vivid as to be painful to the eye.
Some curious facts with regard to flame, in connection with electricity, are given by Brande in the Phil. Trans. for 1814. He supposes that some chemical bodies are naturally in the resinous, and others in the positive electrical state. He supposes also, as a consequence, that the positive flame will be attracted, and neutralize the negative polarity, while the negative flame will operate a similar change by inducing an equilibrium at the positive pole. Thus he found, that certain flames were attracted by the positive ball of an electrical apparatus, and others attracted by the negative ball. The flame of sulphur and phosphorus is attracted by the positive pole, and the flame of camphor, resins, and hydrogen by the negative pole.
In relation to the production of flame, we may observe, that, as sundry solid and fluid substances are inflammable, the products of combustion depend on the composition of the substance made use of, and the condition under which it is burnt. As to gaseous substances that are inflammable, the base of some gases, we may remark, as carbon and hydrogen, unite in the process of combustion with the base of other gases, (as oxygen;) and in other instances, the gas itself takes fire, and exhibits the phenomena of flame. Now carbonic acid gas extinguishes flame, although its base is inflammable; but hydrogen, as well as hydrogen gas, is inflammable, and when burnt in oxygen gas or atmospheric air produces water, which also extinguishes the flame of burning bodies.
As we will have occasion to notice a variety of aëriform fluids, especially when we treat of the aëriform products of fired gun-powder, a few remarks on this head may be useful at this time.
By the combustion of bodies, substances are generated that are either gaseous or solid, whence arises the variety of products. Of aëriform fluids, some are coloured, as nitrous acid vapour, (nitrous gas and oxygen), chlorine, and the protoxide and deutoxide of chlorine. The first is red, the rest yellowish-green, or yellowish. Some relight a taper, provided the wick remain ignited, as oxygen gas, protoxide of azote, and the oxides of chlorine. Others produce white vapours in the air, as muriatic acid, fluoboric, fluosilicic, and hydriodic. The inflammable gases, which take fire in the air by contact of the lighted taper, are hydrogen, hydroguret, and bihydroguret of carbon,