Nitrous oxide gas is a colorless, inodorous gas that can be liquefied by pressure, and is made by heating ammonium nitrate to 350-450° F., and washing the gas. It is used in dentistry, in minor surgery, and as a starter in ether and chloroform anaesthesia. It is administered through an inhaler connected with a cylinder, where the gas is under pressure, and is given without admixture with air. The inhalation is carried on until distinct cyanosis is visible. Unconsciousness comes on in from one-half to three minutes. Anaesthesia occurs when the blood becomes dark. Blood pressure is very high and there is almost complete asphyxia. Return to consciousness occurs in from one to three minutes after removal of the gas, without leaving any after-effects except a slight headache, which may persist for hours. Nitrous oxide is the safest known anaesthetic, and much the most pleasant.