Nitrous oxide, N20, or laughing-gas, is obtained by heating a mixture of salts containing ammonium nitrate. It is marketed under compression in steel cylinders, and is administered by a special inhaler, consisting of a rubber bag and mouth-piece with exit valves for the expired air. It received the name of laughing-gas because in some instances the inhalation of a small quantity of it produced uncontrollable hilarity. A bright, glowing stick plunged into nitrous oxide ionizes it, and bursts into bright flame, as in pure oxygen; but a dull glowing stick goes out and animals and plants quickly die if placed in the gas, for they cannot bring about dissociation to obtain the oxygen. So nitrous oxide will not maintain life, and if used pure, quickly produces asphyxia. It must, therefore, be given with air or oxygen. It has no local action, and, after absorption, exists in simple solution in the blood plasma. But it is not an indifferent gas, like nitrogen, for in 85 or 90 per cent. strength it is a distinct narcotic, capable of producing very rapidly a full degree of unconsciousness, though with incomplete muscular relaxation. Some of the anesthesia has been attributed to asphyxia, but not only is asphyxia not necessary in the anesthesia, but it is to be avoided as much as possible. When air is used as the diluent, there is always some asphyxia, with venous congestion, cyanosis, and raised blood-pressure; so to maintain anesthesia it is now regularly employed with oxygen as the diluent. Teter says that it is impossible to avoid asphyxia with less than 11 per cent. of oxygen.

With the nitrous-oxide-oxygen combination the production of anesthesia is very prompt, and the recovery almost immediate. To produce the anesthesia it may be necessary to add some ether. And it requires such skill to keep the patient in a uniform state of anesthesia of sufficient degree without asphyxia that it is customary to administer, about half an hour before, some slowly acting narcotic, such as morphine sulphate with atropine or scopolamine. Gatch has introduced a method of rebreathing which not only saves gas, but utilizes the patient's own carbon dioxide for the double purpose of stimulating the respiratory center and preventing acapnia. At any time it may be wise to add some ether. The three danger-signals in the administration are vomiting, cyanosis, and slow pulse. A few deaths are reported.

According to Crile, with the same degree of trauma there is only one-fourth as much shock from nitrous oxide as from ether. So the method is an admirable one in the hands of an expert. It is not satisfactory, however, in alcoholics, the obese, and robust athletic persons. It is contraindicated in children under five years, because of the ease with which asphyxia can be produced in such; in old people.with degenerative lesions, because of the high blood-pressure and because of the convulsive movements in case of asphyxia; usually in brain surgery because of increased venous flow; in cardiac weakness because of the raised peripheral resistance; and in any case in which it will not produce anesthesia without cyanosis.

The nitrous oxide and oxygen combination has come into considerable use as the anesthetic of choice for general purposes.

It is especially adapted for short operations and obstetrics. In the latter it may be administered early to produce a form of "twilight sleep," for it is not dangerous to the fetus, does not lessen the strength or frequency of the uterine contractions, and does not predispose to postpartum hemorrhage (Ryder). Nitrous oxide and air are still much employed by dentists in the extraction of teeth, and by anesthetists as a preliminary to ether to avoid the disagreeable first and second stages.