It is not to be expected in every instance, that a rationale of the decomposition as it occurs, or the order in which it takes place, can be given with certainty; because, where a variety of substances enter into the same preparation, which is frequently the case, the affinities become complicated, and the laws of action for that reason indeterminate, and frequently anomalous. But, on the contrary, in a variety of primary operating causes, by which effects analogous in their nature result, decomposition of course being the same, the causes are well understood, and the effects are thereby known, and duly appreciated. This, for instance, is the case with a mixture of nitrate of potassa, charcoal, and sulphur, in the proportion necessary to form gunpowder; for, it is known, that the explosive effects of powder are owing to the sudden production of a number of gases, which suffer dilatation by the immense quantity of caloric liberated at the moment of combustion. Although the production of caloric by the inflammation of gunpowder is a case, which cannot be explained by the present received theory of combustion, as we have noticed in that article; yet we know that it is a fact, and that caloric is generated by the decomposition of the powder. If we consider the primary cause of this decomposition, we naturally inquire into the products of the combustion, and endeavour to account for the production of the elastic aeriform fluids. We know that carbon has the property of decomposing nitric acid, and also nitrate of potassa; for, when it is brought in contact in the state of ignition with nitre, a deflagration will ensue, and carbonic acid be formed. The quantity of this acid is in the direct _ratio_ to the quantity of oxygen required to _saturate_ a given quantity of carbon; and therefore, by employing certain proportions of nitre and charcoal, the latter will decompose the former, and by abstracting its oxygen, on the same principle form carbonic acid, while the azote, the other constituent of nitric acid, will be set at liberty. Nor is this all, if we consider the action of sulphur. The sulphur must unite with one portion of the oxygen to produce sulphurous acid gas, and also with the potassa of the nitre, and form a sulphuret, a compound necessary to be formed, before we can explain the production of sulphuretted hydrogen gas, which results from the decomposition of water contained in the nitre. There also results, as a product, sulphate of potassa. In considering these products at large, it would be necessary to go into detail; and, as we have descanted largely on its combustion in gunpowder, we accordingly refer the reader to the article on _Gunpowder_. It will be sufficient, however, to remark, that the _agent_, and consequently the cause, which produces the decomposition of nitrate of potassa, is carbon or charcoal. This, by uniting with the greater part of the oxygen of the nitre, produces, in a determinate proportion, carbonic acid gas.