To use the instrument, the ethyl chloride vial is removed, and the side pipe attached to a cylinder of nitrous oxide. The indicator of the 3-way tap is put at “air” and the bag mount “in register.” The bag is then filled with gas by opening the head of the cylinder. The bag mount is now put “out of register,” and the side tube disconnected with the cylinder. The bag, being closed, remains full of gas. [Illustration: FIG. 43.--Guy’s Instrument for Gas and Ethyl Chloride. Details of valve piece and bag mount, showing side tube for attachment of Ethyl Chloride vial.] A suitable dose of ethyl chloride is now squirted into the vial. To an adult, Dr Guy gives 3 c.c.: on no account is this dose exceeded: children take 1½–2 c.c.--even adults often get less than 3 c.c. The vial is now attached to the side tube again, and the inhaler is ready for use. After application of the mask to the face, the 3-way tap is at once pushed over to “no valve” and the patient rebreathes the gas in and out of the bag for some six or eight respirations. The bag mount is now turned round “into register,” and the ethyl chloride tipped into the bag. In a further twenty-five seconds the mask may be removed and the operation begun. The available period of anæsthesia is eighty to ninety seconds, counting from the instant of the removal of the inhaler. This method was in use for some years at the Dental Hospital of Edinburgh; no instance of danger to life was ever seen. With so small a dose of ethyl chloride, the erect position necessary for the purposes of dentistry is perfectly safe. [Illustration: FIG. 44.--Diagram of the method introduced by Dr Guy and the author for giving Nitrous Oxide and Oxygen, with or without Ethyl Chloride.] This inhaler, of course, will serve admirably for giving ethyl chloride without gas, and the author habitually uses it for giving the drug by the “vapour” method. In 1911, Dr Guy and the present author modified the method so as to permit the use of oxygen with the nitrous oxide. The inhaler which they then introduced serves also for nitrous oxide and oxygen, unaided by ethyl chloride, and the author has by its means given gas-oxygen to a considerable number of major surgical cases. He now, however, limits its use to short anæsthesias, and uses a sight-feed or a Clarke apparatus for long cases. [Illustration: FIG. 45.--The Guy-Ross Inhaler for Nitrous Oxide and Oxygen, with or without Ethyl Chloride.] In Fig. 44 will be found a diagram showing the method by which the oxygen is introduced. The 1-gallon oxygen bag is either attached directly to a cylinder, or suspended on an upright as shown in Fig. 45. In either case, the bag is, before the administration, moderately filled with oxygen: one bagful will suffice for a short anæsthesia, and the supply of the oxygen from the cylinder is therefore turned off at once. For long cases, of course, a small trickle of oxygen into the bag is required to replace the gas used.