Though not yet recognized by the Pharmacopoeias, nitrous oxide possesses such valuable properties, and is at present so largely employed, that it cannot be passed over in silence in a work, purporting to represent the Materia Medica in its existing state. It was discovered by Dr. Priestly in 1776. but attracted little attention until, in 1800. Si Humphrey Davy made known the remarkable exhilarating property which gave it the name of laughing gas. Even after this time, notwithstanding the excitant powers exhibited by it. and some futile attempts to take advantage of these powers therapeutically, it was scarcely looked on in the light of a remedy until its application, in 1844, by Dr. Horace Wells, of Connecticut, to the purposes of an anaesthetic in dentistry. Since that period, though for a time neglected, it has risen into great importance, partly as a therapeutic agent through the investigations of Dr. Geo. J. Ziegler, of Philadelphia, but chiefly as an anaesthetic, in which capacity, it is due to the dentists to say they have taken the lead in its practical use.

Preparation

The most convenient method of preparing nitrous oxide is by the decomposition, by means of heat, of nitrate of ammonia, which for this purpose should be very pure. The salt is exposed in a glass retort, by means of a lamp or sand-bath, to a heat not exceeding 400° F.; and the gaseous product collected in a glass reservoir, over a saturated solution of common salt, as pure water absorbs a considerable proportion of the gas, and much loss might be incurred in using it. Warm water, however, absorbs much less than cold, and is often employed for the purpose. When nitrate of ammonia is heated, it is entirely decomposed; the hydrogen of the ammonia forming water with three equivalents of the oxygen of the acid, and the nitric oxide thus liberated, combining with the liberated nitrogen of the ammonia to generate nitrous oxide; so that this gas and water are the sole results of the decomposition, if the process is properly conducted. But there are two sources of impurity which must be guarded against In the first place, if too much heat be employed, which will be known by the rising of white fumes in the retort, other products will be evolved, especially nitric oxide, uncombined ammonia, and possibly a little nitric acid; all highly injurious in their effects when inhaled. Secondly, nitrate of ammonia is apt to contain a portion of muriate of ammonia, or other impurity. in consequence of which chlorine may be among the products. To separate these impurities, which would unfit the gas for use, it Should be passed, on leaving the retort, successively through a saturated solution of protosulphate of iron, and a solution of caustic potassa or soda, the former of which will remove the nitric oxide, and the latter. chlorine and whatever acid may be present. The gas thus purified may be kept in gasometers for use; or, as suggested by Dr. Ziegler, it may be forced into water, which can be made to take up five times its bulk, and kept indefinitely in this state. When wanted for use, nothing more will be necessary than to heat the water thus impregnated, which will yield the gas with great rapidity.

Properties

Nitrous oxide is a colourless gas, inodorous, slightly sweetish, and of the sp. gr. 1.527. Water at ordinary temperatures will absorb about three-fourths of its volume, and under pressure will take up much more; and, thus impregnated, has a slightly sweetish taste, and a feeble not unpleasant odour. The gas supports combustion vigorously, and for a time will support respiration, having thus a great advantage over other anaesthetics, which have no power of sustaining life. By the combined influence of cold and pressure, it may be condensed into a liquid, which is colourless, very mobile, and capable of retaining its form, without pressure, at about negative 9° of Fahr. Nitrous oxide consists of one eq. of nitrogen and one of oxygen, and its formula is NO.