The term “CLOSED METHOD” is applied to one in which the patient breathes in and out of a closed bag. The Clover and Ormsby inhalers are “closed” instruments. With this method the patient rapidly uses up the oxygen of the contained air, and accumulates considerable CO_{2}; life could not be sustained for any long period of time under such a system. Oxygen must be supplied from time to time by permitting say one breath in five to be taken from the fresh air instead of from that in the bag. Alternatively, oxygen from a cylinder may be supplied by an accessory pipe into the inhaler. * * * * * This method is also referred to as the RE-BREATHING METHOD. * * * * * The VALVED METHOD is used only with “gas” or “gas-oxygen.” The facepiece fitting accurately, the patient draws all the volume of his inspiration from the inhaler: his expirations he propels through a valve, into the general atmosphere of the room. If nitrous oxide unmixed with oxygen is being given, the patient suffers from oxygen starvation even more rapidly and completely than in the re-breathing method. During the induction period of gas anæsthesia, such oxygen starvation is practised deliberately, and if not pushed too far is harmless. It cannot, however, be continued for more than a brief space of time. The admixture of oxygen to the vapour being breathed entirely abolishes this unfavourable feature of the valved method. There is, however, another consequence of the use of “valves” which is unaffected by the addition of oxygen. Reference has already been made to _Yandell Henderson’s acapnic theory_, and if under any form of anæsthesia the patient can be reduced to a condition _of CO_{2} starvation_, it will be when the valved system of administration is in operation for a prolonged period. As a matter of experience, patients breathing “on the valves” do often exhibit shallow respirations and slight pallor which is rapidly and very strikingly remedied by turning to the rebreathing method. One can hardly doubt that the improvement is due to a gradual re-accumulation of carbon di-oxide in the blood and tissues. Two other terms referring not to the type of inhaler but to the method of supplying the drug, are in use. By the “DROP” METHOD, we mean one in which the anæsthetic is supplied in a steady series of drops. The flow may be quick or slow, but it always arrives on the mask in isolated drops of uniform size. Such a method demands more constant attention than the next to be described, but it is capable of yielding that even uniformity of vapour strength so desirable in open methods. THE DOUCHE METHOD is unfortunately far more commonly used by those whose attention has never been drawn to the significance of the difference between the two. Supplies of the drug rendered, say, every twenty seconds cannot possibly give an even vapour strength.