This section is from the book "Dental Medicine. A Manual Of Dental Materia Medica And Therapeutics", by Ferdinand J. S. Gorgas. Also available from Amazon: Dental Medicine.
N2O. Sp. gr. compared with air is about 1.6.
The discovery of Nitrous Oxide Gas was made by Dr. Priestly in 1776, and scientific investigation demonstrating its respirability, by Sir Humphrey Davy, the results of which were published some twenty years afterward. To Dr. Horace Wells, a dentist of Hartford, Connecticut, is due the credit of having demonstrated its property as an anaesthetic agent for the relief of pain during surgical operations, in the year 1844.
Nitrous oxide is obtained by heating the salt nitrate of ammonia in a glass retort until it melts, and then boils, dissolving into a vapor of water and into a permanent gas. The salt nitrate of ammonia is obtained in three forms - the crystallized, the granulated and the fused. The fused form is prepared by melting the crystallized nitrate of ammonia, and allowing it to solidify on cooling; the granulated is prepared by evaporating the solution to a density sufficient to solidify on cooling, and agitating the mass until it becomes cool.
As it is very important to use a pure form of the nitrate of ammonia in the generation of nitrous oxide gas, the salt, either in the fused or granulated state, may be tested by heating it on platinum, when the nitrate, if pure, should volatilize perfectly. It may also be tested by a few drops of chloride of barium, which should not give a precipitate when added to the nitrate of ammonia; should it do so, then sulphuric acid is present; or the nitrate of ammonia may be tested by a solution of nitrate of silver, which will also give a precipitate, if chlorine is present. The test with nitrate of silver may be made by dissolving a drachm or the nitrate of ammonia in half a glass of distilled water and adding a few crystals of the nitrate of silver. If the nitrate of ammonia is pure, the solution will remain perfectly clear; but should chlorine be present, it will show a clouded appearance, and should be discarded, as it is not fit to generate the gas from.
The salt nitrate of ammonia is a white, crystalline substance, formed by neutralizing dilute nitric acid by means of carbonate of ammonia, and is so constituted as to be resolved into nitrous oxide and water. A pound of the salt will generate about thirty gallons of the gas. In generating nitrous oxide gas, care must be taken to preserve its purity; consequently it becomes important to maintain, as nearly as possible, an equal temperature during its manufacture; and should any nitric oxide - binoxide of nitrogen - be formed, which may be done by too high a heat under the retort, it is necessary to remove it and other impurities, by the aid of solutions of caustic potash and copperas; the caustic potash neutralizing any free nitric acid present, and the copperas removing either chlorine or nitric oxide. The salt nitrate of ammonia is perfectly fused at 2260 F., emits white fumes at 3020 F., and begins to evolve gas at 4600 F. At 5000 F. the impurity, nitric oxide, is given off; hence the temperature during the process should not be permitted to rise above 4820 F., which can be determined by thermometers prepared for the purpose. No red fumes should pass from the retort.
Nitrous oxide gas, as principally used at the present time, is in the form of condensed gas, being liquefied and solidified under great pressure, and secured in iron cylinders, from which it escapes into an inhaling bag when needed for use by turning a stop-cock. Liquefied nitrous oxide is a colorless fluid, having specific gravity of 0.908, and at a very low temperature (1480 F.) it congeals into a transparent crystalline solid. It is prepared by condensing the gas by a pressure of fifty atmospheres at a temperature of 400 F., and may be kept in the iron cylinders for an indefinite period. The advantages of the condensed form of the gas are its purity, convenience for use whenever needed, the large supply which can be kept for use, and the freedom from deterioration, notwithstanding its age.
Nitrous oxide is an elastic, colorless gas, with a faint, but agreeable odor and a sweetish taste, which it imparts to water. Under a pressure of fifty atmospheres at about 400 F., it becomes a clear, colorless liquid, and at about 1480 F. below zero, it freezes into a beautiful, clear crystalline solid. By the evaporation of this solid, a degree of cold may be produced far below that of carbonic acid bath in vacuo, or lower than 1700 F. The washing of nitrous oxide gas, and the retention of it over water for any length of time, is attended with considerable loss, as much of it is absorbed, especially if the water is cold; such loss may be avoided to a great extent by using water of an elevated temperature or a strong solution of common salt. The impurities of nitrous oxide gas are air, water, nitric oxide or binoxide of nitrogen, chlorine and hyponitric acid. The mixture with air which may occur in the receiver, or when the patient is inhaling the gas, merely dilutes it as does also the vapor of water. Nitric oxide, however, is a dangerous impurity, and with others may be generated even when pure nitrate of ammonia is used in preparing the gas; this impurity is also one of the most difficult to separate from the gas. Like nitrous oxide, nitric oxide is a colorless gas, lighter and less soluble in water, and if it is generated is liable to pass through the washing solutions into the gasometer or receiver; nitric oxide, however, need not be generated if the proper care is observed with regard to the degree of heat under the retort which contains the nitrate of ammonia.
Chlorine is also a dangerous impurity, and may be set free if the salt nitrate of ammonia contains muriate of ammonia (sal-ammoniac) and chloride of ammonium. To prevent such contamination, the tests of the salts before referred to may be made. Nitrous oxide gas supports combustion with almost the same promptness as pure oxygen; and although nearly identical in constitution with atmospheric air, it differs from it both in the proportion of its constitutional elements and in the manner of their association. Nitrous oxide contains about one-third of oxygen to two-thirds of nitrogen, while atmospheric air has only about one-fifth of oxygen to four-fifths of nitrogen.