Most commonly nitrous oxide is administered to a patient sitting in a chair. Care should be taken that the respiration of the patient shall not be obstructed by tight clothing round the throat or chest, and that the head and neck are neither unduly flexed nor extended upon the shoulders. The patient should not have any solid food for two hours before the anæsthetic. At the last moment he should be instructed to empty his bladder. Artificial dentures if present should be removed, and, if the anæsthetic is being given for the purpose of the extraction of a tooth, it will be necessary before applying the facepiece to insert between the teeth a dental prop. (See Fig. 6.) Standing to the left and slightly behind the patient the anæsthetist’s first step is to secure good apposition between facepiece and face. This is best done by working from above downwards: that is to say, secure first a good fit at the bridge of the nose, and then approximate the remainder of the rim of the facepiece to the cheeks and lower jaw. During this stage the indicator of the tap is kept at “Air.” Working with the left foot the administrator now opens the head of one of the cylinders with the foot key. It is wise first to have loosened this with a hand key, and leave it just “on the swing,” otherwise one’s boots are apt to suffer! Gas is allowed to flow into the bag until it is partially, but by no means tightly distended. The patient is instructed to breathe naturally and easily, and during the whole process the anæsthetist should converse with him in a quiet easy way. The tap is now turned to “valves” and the patient begins to inspire the gas, a supply of which is allowed to flow steadily from cylinder into bag. After a few breaths of the gas, when the sensibilities of the patient are a little dulled, it is wise to allow the gas to flow a little more freely and to distend the bag. This exercises upon the patient a slight _positive pressure_, which has been proved both experimentally and practically to increase the rapidity of the absorption.[1] The valves are left in operation for some thirty or forty seconds, after which time the supply of gas should be cut off and the tap be pushed over to the position of “no valves” for further twenty seconds. This should be ample to secure full gas anæsthesia.