Idiosyncracies of persons have much to do with lesions of the teeth, but there are often other causes also. A patient comes from some other operator, and ignorant of what has been done, he is apt to be misled. All causes which are not directly associated with the trouble, if first eliminated, the diagnosis is much easier. The prejudice and ignorance of the patient must be considered. A previous condition should be studied, and its influence at the present time weighed that we be not led to error. We must not be influenced by mere probabilities. Teeth once affected are quite likely to be mistaken as the cause of trouble again. _Discussion_—DR. OTTO ARNOLD:—Instinct is a pretty good guide in difficult diagnosis. DR. R. D. JENNINGS:—Physicians know very little about the teeth. DR. M. H. FLETCHER:—Pulp stones are often the cause of lesions very difficult of diagnosis. Dr. L. E. Custer, of Dayton, O., read a paper upon "Chloride of Methyl." Since Dr. B. A. R. Ottolengui introduced the ether spray for obtunding sensitive dentine there have been a number of such agents introduced for the same purpose. There has been considerable confusion regarding the action of volatile agents for sensitive dentine. Dr. Ottolengui in previous paragraph first used ether, supposing it to dehydrate further than alcohol or hot air; but it does not, because it has no affinity for water and because it produces cold, which opposes vaporation of the dentine's moisture. Dr. Curtiss in using nitrous oxide is led to the belief that it obtunds because it dehydrates. But that is a mistake, for the same reason as for ether. When these agents obtund sensitive dentine, it is because they reduce the temperature far below normal, and the crown of the tooth being easily isolated, it is not difficult to reduce the temperature so low as to render it entirely void of sensibility. Chloride of methyl being an ether quite low in the series, volatilizes at 74° Fahr. between ether and rhigolene, and is capable of reducing the temperature to 40° below zero, if necessary. It volatilizes with considerable rapidity with the warmth of the hand, and so does away with any blast apparatus. Is not as inflammable as ether. Is not a solvent of caoutchouc, and so it may be conducted through a rubber tube to the tooth and its flow regulated by a thumb-screw. It volatilizes quicker than ether, so that the surrounding parts are not saturated. Its objections are that it is a general anæsthetic and at the present time rather costly. It is all that Dr. Rhein claims for it, and is to be preferred as a cold-producing agent for sensitive dentine. But the more experience we have the better we can control our patients by a sort of personal magnetism, and dehydration will answer our purpose without resorting to such dangerous methods as the reduction of temperature. _Discussion_—DR. C. R. BUTLER:—General anæsthesia is probably slightly produced, which accounts for part of the effect.