The gas produced from fibrous nitrate, must be suffered to rest at least for an hour after its generation. At the end of this time it is generally fit for respiration. If examined before, it will be found to contain more or less of a white vapor, which has a disagreeable acidulous taste, and strongly irritates the fauces and lungs. This vapor, most probably, consists of acid nitrate of ammoniac and water, which were dissolved by the gas at the temperature of its production, and afterwards slowly precipitated. It is found in less quantity when compact nitrate is employed, because more salt is sublimed in this process, which being rapidly precipitated, carries with it the acid and water. Whatever salt is employed, the last portions of gas produced, generally contain less vapor, and may in consequence be respired sooner than the first. The nitrate of ammoniac should never be decomposed in a metallic vessel,[95] nor the gas produced suffered to come in contact with any metallic surface; for in this case the free nitric acid will be decomposed, and in consequence, a certain quantity of nitrous gas produced. [95] Except it be gold or platina. The apparatus that has been generally employed in the medical pneumatic institution, for the production of nitrous oxide, consists 1. Of a glass retort, of the capacity of two or three quarts, orificed at the top, and furnished with a ground stopper. 2. Of a glass tube, conical for the purpose of receiving the neck of the retort; about ,4 inches wide in the narrowest part, 4 feet long, curved at the extremity, so as to be capable of introduction into an airholder, and inclosed by tin plate to preserve it from injury. 3. Of airholders of Mr. Watt’s invention, filled with water saturated with nitrous oxide. 4. Of a common air-furnace, provided with dampers for the regulation of the heat. The retort, after the insertion of the salt, is connected with the tube, carefully luted, and exposed to the heat of the furnace, on a convenient stand. The temperature is never suffered to be above 500°. After the decomposition has proceeded for about a minute, so that the gas evolved from the tube enlarges the flame of a taper, the curved end is inserted into the airholder, and the nitrous oxide preserved. The water thrown out of the airholders in consequence of the introduction of the gas, is preserved in a vessel adapted for the purpose, and employed to fill them again; for if common water was to be employed in every experiment, a great loss of gas would be produced from absorption. A pound of fibrous nitrate of ammoniac, decomposed at a heat not above 500°, produces nearly 5 cubic feet of gas; whilst from a pound of compact nitrate of ammoniac, rarely more than 4,25 cubic feet can be collected.