(4) _Re-breathing._--To supply the whole volume of gases required for inspiration during a long operation is costly and quite unnecessary. Yet that is what is being done if the whole administration is conducted upon the “valvular” principle. Moreover, a prolonged inhalation upon the valves tends to remove a great deal of CO_{2} from the patient’s blood and tissues (_see_ Chap. IV.). Periods of partial or complete re-breathing do much to deepen respiration, and reduce the cost of the anæsthetic. (5) _Warming the Gases._--While not essential, this is certainly an advantage. (6) _Addition of Ether Vapour to the mixture._--Gas-oxygen even well given is hardly capable of reducing to quiescence very robust people, unless the oxygen percentage is kept to an undesirably low level. The merest trace of ether vapour as an adjunct is a great assistance during the stages of the operation where very sensitive structures such as the parietal peritoneum are being handled. The more experienced the anæsthetist, the less will he require such assistance. Hewitt’s Apparatus. This, with the exception of one designed by Dr Guy and the author, and described on page 130, is the only machine with any pretence to portability by hand. It does not satisfy all the requirements above referred to, but the fact that it was the first practicable means introduced in this country to give gas-oxygen entitles it to full description. (_See_ Fig. 18.) [Illustration: FIG. 18.--Hewitt’s gas oxygen apparatus.] Essentially it consists of the following:-- (1) A supply of nitrous oxide and oxygen in separate cylinders. Hewitt’s own stand held two of nitrous oxide and one of oxygen.[3] (2) Rubber pipes of supply for each of the two gases. For convenience, it is sometimes arranged that one of these shall run inside the other. (3) Two 2-gallon bags. Nitrous oxide is led into the one, oxygen into the other. The mouth of _each_ bag is guarded by an inspiratory valve. (4) The mixing chamber. Upon the surface of this are marked successively: “Air,” “N_{2}O,” “O_{2}, 1 2 3 4 5 6 7 8 9 10.” As the indicator is pushed from “air” to “N_{2}O,” the patient begins to inhale nitrous oxide only, but as it travels into the numerals 1, 2, etc., a proportion of oxygen is added. Immediately below the mixing chamber is the _expiratory valve_. (5) Lastly, there is the _facepiece_, identical with that used for pure nitrous oxide.