It is known that sulphuric acid forms, with lime, an almost insoluble compound, called sulphate of lime, or gypsum; and hence, when sulphate of potassa is mixed with a solution of nitrate of lime, nitrate of potassa is formed, which remains in solution, and sulphate of lime is precipitated. The same effect takes place with all earthy nitrates. For the application of sulphate of potassa, in this way, we are indebted to M. Berard. It might be advantageously employed in decomposing the calcareous nitrate of the nitre-caves of the western country. M. Longchamp has recommended the use of sulphate of soda, or Glauber's salt, for decomposing the muriate of lime, which exists occasionally in impure nitre. These two salts reciprocally decompose each other; sulphate of lime is precipitated, and muriate of soda remains in solution. The latter is separated by evaporating the nitrous solution. M. de Saluces (_Mémoire de l'Académie des Sciences de Turin, Année, 1805 à 1808_,) has proposed a new process for purifying nitre. It consists in filtering it through argillaceous earth, or clay. Although the process is highly spoken of, yet we can see no particular advantage it possesses. Chaptal observes, that the process mostly in use is that of dissolving 2000 pounds of crude saltpetre in a copper boiler, in 1600 lbs. of water. As the solution is made by the heat, the scum, which forms, is taken off. Twelve ounces of glue, dissolved in ten pints of boiling water, and mixed with four pails full of cold water, are then added. This addition cools the solution. As to the manipulations of the process, they have been given. The principal thing to be attended to, is to separate the marine salt, which is done during the boiling. To pass this saltpetre through a second operation, in order the more to purify it, it is again dissolved, in the proportion of 2000 pounds, in one-fourth of its weight of water. Heat is applied. The scum is separated; a solution of 8 ounces of glue in one or two pails full of water is then added. After the solution becomes clear, it is suffered to cool, and at the expiration of five days, it will crystallize, or form in a mass, which is then exposed to the air six or eight weeks to become completely dry. In treating of the formation of nitre in France, Bottée and Riffault (_Traité de l'Art de Fabriquer la Poudre à Canon_,) consider it under the following heads: 1. _The constituent principles of nitre; its generation, and the theories respecting it._ In this article, the composition of nitric acid and its union with potassa, and the production of artificial nitre, are taken into view. 2. _Nitrous earths, and substances which yield saltpetre._ This subject comprehends a view of the substances, which contain saltpetre, as well as those which afford it by nitrification. 3. _The preparation of the substances to produce saltpetre._ This article relates to the manipulations required for the production of nitre.