The above, then, may be taken as an account of what one expects to see in a normal gas anæsthesia during the induction stage. For the great majority of cases, nitrous oxide is given for the purpose of rendering painless the extraction of a tooth, and it is, in this large class of case, the induction stage only which need be considered. It requires only some fifty to sixty seconds to bring the patient to the stage described under the heading “fully developed anæsthesia” and when that has been attained, the mask may be removed and the operation begun. From the moment of removal of the mask, however, it must be noted that the patient begins to breathe fresh air and to eliminate the N_{2}O. The period of anæsthesia available to the surgeon or dentist during which he must perform the operation, is therefore very small. In thirty seconds the patient has frequently recovered sufficiently to begin to feel pain, and it is rare to secure more than forty-five or fifty seconds by the use of a single dose of nitrous oxide. Nitrous Oxide and Air. If the nature of the operation does not necessitate the removal of the mask from the face, it is possible to maintain nitrous oxide anæsthesia for some considerable time. The exact length of that time varies a good deal with two factors--the type of patient and the experience of the administrator. Heavily built muscular patients are not easily dealt with by prolonged gas anæsthesia (unless with admixture of oxygen as explained in chapter viii.) Of far greater importance, however, is the other factor. The student can easily be taught to give a single dose of gas for the extraction of a tooth, or the momentary incision of an abscess. He will, however, be wise to secure a good deal of practice in that class of work before attempting to prolong gas anæsthesia for more than a minute or two.