Saltpetre, when properly refined, does not contain any foreign salts, and its purity may be known by a variety of experiments, as follows: make a solution of the salt in distilled water, and filter it through paper. Put a portion of it in a wine glass, and add a solution of carbonate of potassa. To another portion, add a small quantity of muriate, or in preference, nitrate of barytes. To a third portion, add nitrate of silver. If the fluid in the first glass remains clear, without any turbidness, we are to infer the non-existence of earthy salts; if turbid, that it contains lime, or some other earth, either in the form of a nitrate or muriate. The addition of oxalate of potassa to another portion of the solution will show the presence of lime by forming a precipitate, and the addition of carbonate of ammonia, and then of phosphate of soda, will indicate magnesia. If the second glass remains transparent, it shows that neither sulphuric acid, nor any of the sulphates are present. If the fluid in the third glass continues also clear, we infer that none of the muriates exist. These experiments are sufficient to show the purity of saltpetre. It would afford perhaps more satisfaction to institute also the same experiments on other samples of nitre, by which a comparison may be formed of the relative purity of each. To make an analysis of the salt, with the view to determine the proportion of the foreign substances would be altogether unnecessary for common purposes. A regularly defined crystal would, in a great measure, point out its purity. The double refined saltpetre is chemically pure. Artificers determine the purity of nitre by its flame; if white, they call it pure, if yellow, impure. The same reagents may be used in the examination of gunpowder, as we shall notice hereafter. If a portion of powder be mixed with distilled water, the water will dissolve only the saline substances, leaving the charcoal and sulphur. When the whole is thrown on a filter, the fluid, which passes through, will contain the saltpetre, and foreign salts, if any are present. The same experiments may then be performed with the solution, and the quality of the nitre, of which the gunpowder was made, be determined. Some gunpowder absorbs a large portion of water, which is owing to the presence of deliquescent salts. These salts may be detected by proceeding in the way we have pointed out. The art of refining saltpetre is so well known of late in the United States, especially by the Messrs. Dupont of Brandywine, Delaware, that our gunpowder is of a very superior quality. I have examined various specimens of this saltpetre, and gunpowder made with it, and could not detect any of the sulphates or muriates, either alkaline or earthy. For the manufacture of gunpowder, and fire-works generally, the nitre, it may be observed, cannot be too pure.