As regards (3) and (4), the lack of toxic properties in nitrous oxide gas, and the rapidity with which it is eliminated, give it a tremendous advantage over ether or chloroform. To men with shattered bones and extensive damage to soft tissues, badly infected with sepsis, who required repeated opening up of pockets, changing of gauze packs, etc., the advantage of gas-oxygen over ether was evident, and was easily appreciated by the patients themselves during the late war. There are, however, certain _drawbacks_ to the method which must be appreciated-- (1) The necessary plant is heavy, bulky, and costly; it cannot be easily transported. (2) The running cost is high as compared with ether or chloroform. (3) It has been said by some that gas-oxygen can only be given by an expert. That is a statement too extreme, in the author’s opinion. Certainly, of all anæsthetics it is the most difficult to give successfully. Adequate study and proper teaching by an expert are required, but given these two helps, any one can soon learn to administer gas-oxygen for minor surgery. Considerably more experience is, however, necessary before the beginner should give it for an abdominal section. Apparatus. A good gas-oxygen apparatus is necessarily rather complicated. The machines in the market are numerous, and of the most diverse external appearance. Certain broad principles, however, underlie all the machines, and it is to be hoped that some one of them will before long become practically the standard. Once that is effected, hospitals and nursing homes could be expected to provide them. So long as every anæsthetist asks for a different machine, they certainly never will do. A good machine must provide means for the following:-- (1) _An even flow_ of both gases under perfect control. (2) _A percentage of Oxygen_ in the mixture rising at the will of the administrator from 2 to 15 or 20.[2] To meet this requirement it is not necessary that any indicator should be provided which shows with mathematical precision what percentage of oxygen is being given. The colour of the patient tells us at once if too much or too little oxygen is being supplied, and all we need in the apparatus is some mechanism whereby we can tell approximately to what extent we are increasing or decreasing the percentage. (3) _Positive Pressure._--If the pressure at which the gases are supplied to the patient can be raised a little above that of the ordinary atmosphere, absorption is increased and a deeper anæsthesia produced. In the author’s view, this is an essential point in a good instrument.