The active principle which produces this result is the crystalline alkaloid, the _theine_,[18] a compound belonging to the same class as strychnine and a number of similar vegetable poisons. These, when diluted, act medicinally—that is, produce disturbance of normal functions as the tea does, and, like theine, most of them act specially on the nervous system; when concentrated they are dreadful poisons, very small doses causing death. The volatile oil, of which tea contains about 1 per cent., probably contributes to this effect. Johnston attributes the headaches and giddiness to which tea-tasters are subject to this oil, and also ‘the attacks of paralysis to which, after a few years, those who are employed in packing and unpacking chests of tea are found to be liable.’ As both the alkaloid and the oil are volatile, I suspect that they jointly contribute to these disturbances, the narcotic business being done by the volatile oil, the paralysis supplied by the alkaloid. The non-tea-drinker does not suffer any of the five-o’clock symptoms, and, if otherwise in sound health, remains in steady working condition until his day’s work is ended and the time for rest and sleep arrives. But the habitual victim of any kind of drug or disturber of normal functions acquires a diseased condition, displayed by the loss of vitality or other deviation from normal function, which is temporarily relieved by the usual dose of the drug, but only in such wise as to generate a renewed craving. I include in this general statement all the vice-drugs (to coin a general name), such as alcohol, opium, tobacco (whether smoked, chewed, or snuffed), arsenic, haschisch, betel-nut, coca-leaf, thorn-apple, Siberian fungus, maté, &c., all of which are excessively ‘refreshing’ to their victims, and of which the use may be, and has been, defended by the same arguments as those used by the advocates of habitual tea-drinking. Speaking generally, the reaction or residual effect of these on the system is nearly the opposite of that of their immediate effect, and thus larger and larger doses are demanded to bring the system to its normal condition. The non-tea-drinker or moderate drinker is kept awake by a cup of tea or coffee taken late at night, while the hard drinker of these beverages scarcely feels any effect, especially if accustomed to take it at that time. The practice of taking tea or coffee by students, in order to work at night, is downright madness, especially when preparing for an examination. More than half of the cases of breakdown, loss of memory, fainting, &c., which occur during severe examinations, and far more frequently than is commonly known, are due to this. I continually hear of promising students who have thus failed; and, on inquiry, have learned—in almost every instance—that the victim has previously drugged himself with tea or coffee. Sleep is the rest of the brain; to rob the hard-worked brain of its necessary rest is cerebral suicide.