Fig. 15 shows the ordinary _Barth three-way_ tap with facepiece; the indicator on the tap has three possible positions designated on the dial as “Air,” “Valves,” and “No Valves.” If the tap is pointed backwards towards the bag at the position marked “air” the end of the bag is closed and the patient is breathing air only. With the tap in the middle position of “valves,” the inspiration of the patient will draw gas from the bag, but the expiration closes the valve which is now in operation at the orifice of the bag, and will open the expiratory valve which conducts the expired air into the general atmosphere. In the third position of “no valves” the patient breathes both in and out of the bag. [Illustration: FIG. 15.--Barth 3-way N_{2}O tap.] [Illustration: FIG. 16.--Hewitt’s wide-bore gas valves.] Fig. 16 shows the Hewitt type of inhaler. The calibre of the orifices through which respiration takes place is greater than in the Barth three-way tap, and to that extent this type of valve is to be preferred. Although differently arranged exactly the same possibilities are present in it. The _facepiece_ is sometimes made of celluloid with an inflatable rubber edging. The object of this type of facepiece is that the colour of the lips may be appreciated by the anæsthetist during the administration. The preferable plan is to make the whole facepiece of rubber with an inflatable border. Such a facepiece made by a good maker will last many years, and is much more stable and reliable than its celluloid competitor. The Care of the Nitrous Oxide Apparatus. This is a matter of considerable moment, particularly to those who do not use their apparatus every day. After use, the valves, facepiece, and bag should be disconnected from each other, all moisture wiped away from the bright parts, and the bag hung up with its open end downwards, and preferably in a warm room. If re-breathing has been extensively practiced it is well to wash the bag out with some carbolic lotion before hanging it up. The rubber valves in the valvepiece are liable to lose their elasticity, particularly if kept in a cold place after becoming damp. From time to time the valve piece should be taken to pieces, the valves carefully dried in front of a warm fire and powdered over with a little talc. Administration.