The active principle _Nicotine_ was first isolated in 1828, by Posselt and Reimann, and is an almost colourless, oily liquid of a highly poisonous nature. It soon becomes brown on exposure to air or light. The amount present in tobacco leaves varies considerably, but it is usually about six per cent. It has not been met with in tobacco smoke, according to Vohl, but the tobacco oils contain minute proportion of nicotine. One drop of pure nicotine is sufficient to kill a dog, while a very little more will destroy life in a human being. It is said to possess the property of resisting decomposition amid the decaying tissues of the body, and was detected by Orfila two or three months after death. Vohl and Eulenberg have made an interesting investigation of tobacco smoke. The smoke analysed was from a tobacco containing four per cent. of nicotine, but none of the alkaloid was found in the smoke. In the smoke of cigars certain gases were given off, and an oily body collected, which, on distillation, yielded aromatic acids. Distilled at a temperature above boiling water, tobacco gives an empyreumatic oil of a poisonous nature. It exactly resembles that which collects in the stems of tobacco pipes, and contains a small percentage of nicotine. The actual amount of nicotine absorbed into the blood while smoking a pipe is very minute, at least fifty per cent. of the entire alkaloid being destroyed by decomposition, and escaping from the bowl of the pipe. The habitual inhalation of tobacco smoke is undoubtedly harmful, but unless the smoke be intentionally inhaled, very little makes its way into the lungs. A great deal of misconception exists in the mind of the average individual as to the power of the alkaloid of tobacco. The amount of nicotine actually absorbed from a fair-sized pipe is about one-fortieth of a grain, in a cigar rather less. Death has resulted after smoking eighteen pipes, and from twenty cigars smoked continuously.