AddendumSince this paper was read it has been established (Newton-Andrews, 1954; Bourne, 1954) that under certain conditions it is possible to strike sparks when dental forceps slip. Cyclopropane should not, therefore, be used in aneesthesia for extraction of teeth until this matter has been fully investigated (Bourne, J. G., 1954, Brit. med. J.-, i, 937; Newton-Andrews, H., 1954, Brit. med. J., i, 641).CRITICAL REVIEW OF NITROUS OXIDE NITROUS OXIDE has long been the mainstay of anesthesia for out-patients. I have criticized this use of nitrous oxide, arguing that its low potency, usually regarded as an advantage, is, in fact, a disadvantage. Contrary to current belief, a narcotic of high potency, I suggested, might be not only more effective but also safer, and my experience with cyclopropane led me to think that more nearly than any other agent it met the requirements of out-patient work (Bourne, 1952).Continuing this study, I thought it worth while to make a fresh clinical assessment of the potency of nitrous oxide. For this purpose I anesthetized 60 unselected adult in-patients in the following way: They were premedicated with hyoscine, or, if they were over 60, with atropine. Anesthesia was induced with 50% cyclopropane in oxygen breathed for thirty seconds from a 6 litre bag. During this inhalation suxamethonium (Scoline) was injected and as soon as the jaw relaxed a cuffed tube was inserted into the trachea. Anxsthesia was then continued with nitrous oxide and oxygen, given at first by artificial respiration, and time was measured from the start of this inhalation.