PREPARATIONS 299 becomes semi-solid owing to the separation of aniline sulphate, but later becomes mobile as the temperature rises during the further addition of the acid. Now add some fragments of un-glazed porcelain, fit the reflux condenser to the flask, and support the latter on a gauze-covered tripod, preferably in a fume-cupboard; clamp the condenser so that it is inclined at about 60° to the horizontal, as "choking" is less likely in this position than in a vertical position. Now heat the flask cautiously with a Bunsen flame: immediately the reaction starts and bubbles of vapour form in the liquid, turn out the gas. (It is advisable to have a duster soaked in cold water at hand: if, when the reaction starts, the duster is wrapped around the upper part of the body of the flask, the risk of the condenser choking becomes negligible.) When the vigorous reaction subsides, replace the gauze by a sand-bath and boil the mixture gently for 3 hours. By this time the reaction is complete, and no unchanged nitrobenzene should
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