The experimental background for encephalography with the use of anesthetic gases has been reported in previous articles.1 The characteristics of the severe and prolonged reaction following the injection of air and the attempts of various workers to find a suitable substitute were discussed. The use of oxygen by Jüngling2 in 1922, by Denk3 in 1923 and more recently in Penfield's clinic,4 as well as the trial of carbon dioxide, nitrogen and helium by Liberson,5 were mentioned. A review of the literature on the experimental trial of various opaque fluids was also presented. A theoretical consideration of the various factors involved in the problem of encephalography led to the establishment of certain criteria. In this connection it occurred to me that an anesthetic gas might approximate the ideal desired. In brief, it seemed possible that even potent anesthetic gases might prove safe, since only a limited