Nitre, or saltpetre, is one of a class of substances which contains a large portion of oxygen in a combined and solid state; and, being mixed with combustible matter such as charcoal, it causes rapid deflagration when the temperature is raised. The whole class of pyrotechnic compositions are reducible to this simple principle--they all consist of combustible substances intimately mixed with substances containing oxygen; or, to reduce the proposition to more general and simple terms, they consist of two or more substances, having for each other a powerful chemical affinity, and capable of rapidly uniting when the temperature is elevated. When a projectile force is necessary, a further condition is essential, viz., that they liberate by their chemical action gaseous matter, whereby a sudden increase in volume is produced, the expansion of which, augmented by the high temperature, produces the required effect of propulsion. This slight sketch will show that the purity and proportions of the saltpetre, and the inflammable substances mixed with it, are the main elements to be attended to in the improvement of self-burning compositions: it is indeed far from improbable, that the substances used in purifying saltpetre have first suggested such compounds. Wood ashes were used at a very early period for purifying nitre; and at the end of an Arabic receipt of the thirteenth century, for the preparation of saltpetre, in which charcoal is used, is the expression, "guard against sparks of fire." The probabilities strongly favour the view, that incendiary compositions of the nature we have been describing originated with the Chinese. China snow, and China salt, are the names given by writers of the greatest antiquity to saltpetre. In the Arabic MSS. to which we shall presently allude, the words Chinese wheel, Chinese flower, Chinese dart, occur as appellatives of different fireworks. It is very possible that the influx of Chinese literature, which the result of the recent war with that people promises us, will lead to the discovery of Chinese treatises upon pyrotechny. Other authors speak of fire-arms among the Chinese at a very early period of our era, and even before Christ; but the interpretation which they have put upon obscure passages--interpretations evidently derived from their existing knowledge--makes these expressions and translations of extremely doubtful import. At a later period, however, we have the authority of Raschideddin, (minister of the Tartar Khan of Persia,) and of Marco Polo, that the machines of war employed at the siege of Siang Yang were constructed by Arabian or European workmen, and that the Tartars were not at this period themselves able to manufacture such machines. This would tend to negative the belief which has been entertained by some, that the Chinese then used gunpowder as a means of projection, but does not lessen the possibility that the fuses and compositions projected by these machines were of Chinese origin.