If the object be the removal of a tooth, the mask may now be removed, and the dentist may rely upon a period of anæsthesia somewhat longer than that furnished by pure nitrous oxide. He ought to secure approximately one minute in which to do his work. Anæsthesia by this apparatus may however be prolonged for an indefinite time if desired. In order to maintain the patient in the condition described above, it will be necessary gradually to increase the supply of oxygen. For this purpose Hewitt added to his mixing chamber a supplementary oxygen supply giving 10 or 20 volumes of oxygen. As a matter of fact, all the necessary supply can be got through the original ten holes _if the tension in the oxygen bag be increased_. The regulation of all this requires, of course, considerable practice and experience. The signs upon which we rely for warning that the supply of oxygen is insufficient to keep the patient safe, are chiefly the colour of the face, which must not pass from dull pink to blue, and the size of the pupil. [Illustration: FIG. 19.--Diagrams to illustrate action of (A) Hewitt (B) Teter gas oxygen apparatus. Note that in A the mouths of both bags are guarded by valves of inspiration, while in B the oxygen bag only possesses it.] Deficiencies of Hewitt’s Apparatus. These are chiefly two:-- (1) There is no means of producing _positive pressure_. Any attempt to distend the nitrous oxide bag beyond a certain point simply leads to escape of the gas through the mixing chamber and out of the expiratory valve even during inspiration. To this defect especially must we attribute the fact that an anæsthesia deep enough for abdominal section is difficult to secure with the Hewitt instrument. (2) There is no means of securing _re-breathing_. The whole administration must of necessity be conducted “upon the valves.” This latter fault is remedied by the modification introduced by _Burns_, who took away the inspiratory valve from the mouth of the N_{2}O bag, and fitted a cap over the expiratory valve which could be rotated so as to throw the valve out of action. The author first met this modification at a Base Hospital in France, and found it a great improvement upon the original instrument. It is, however, only an imperfect attempt to adopt Teter’s chief principle. The Teter and Allied Machines. The rapid spread of nitrous oxide and oxygen anæsthesia in the U.S.A. brought forward a number of machines of which Teter’s was the forerunner; the other well-known machine of the group is the Clarke. They differ in principle from the Hewitt apparatus in that they permit re-breathing and the use of positive pressure. Diagrammatically, the two are contrasted in Fig. 19.