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.