A distinguished physician and man of science, Sir B. W. Richardson, has tested the tobacco leaf and all its component parts with a thoroughness which puts to flight all doubts as to what it is ‘men put into their mouths to take away their brains.’ The chief results of his experiments may be briefly summarised. Although evident differences prevail in respect to the products arising from different cigars, different tobacco, and different pipes, there are certain substances common to all varieties of tobacco-smoke. Firstly, in all tobacco-smoke there is a certain amount of watery vapour which can be separated from it. Secondly, a small quantity of free carbon is always present; it is to the presence of this constituent that the blue colour of tobacco is due. It is this carbon which in confirmed and inveterate smokers settles on the back part of the throat and on the lining of the membrane of the bronchial tubes, creating often a copious secretion which it discolours. Thirdly, the presence of ammonia can be detected in small quantity, and this gives to the smoke an alkaline reaction that bites the tongue after long smoking; it is the ammonia that makes the tonsils and throat of the smoker so dry, and induces him to quaff as he smokes, and that partly excites the salivary glands to secrete so freely. This element also exerts an influence on the blood. Fourthly, the test of lime-water applied to the leaf shows the presence of carbonic acid. In the smoke the quantity differs considerably in different kinds of tobacco; to the action of this constituent Sir B. W. Richardson traces the sleepiness, lassitude, and headache which follow upon prolonged indulgence of the pipe. Fifthly, the smoke of tobacco yields a product having an oily appearance and possessing poisonous properties; this is commonly known as nicotine, or oil of tobacco, which on further analysis is found to contain three substances, namely, a fluid alkaloid (the nicotine of the chemist), a volatile substance, having an empyreumatic odour, and an extract of a dark resinous character, of a bitter taste. From this comes the smell peculiar to stale tobacco which hangs so long about the clothing of habitual smokers—if the smell be from good Eastern-grown tobacco many persons think it wholesome. It is nevertheless this extract which creates in those unaccustomed to its use a feeling akin to sea-sickness. Hence it appears that the more common effects are due to the carbonic acid and ammonia liberated in the process of smoking, while the rarer and more severe symptoms are due to the nicotine, the empyreumatic substance, and the resin.