Caffeine, the active principle in coffee, first extracted by Runge in 1820. It is a weak alkaloid, identical in chemical composition with theine, the active principle of tea. Being found in all the varieties of coffee as well as of tea, which are used as drinks by a large portion of the human race, it no doubt possesses some properties of importance to the animal system. Very few other substances contain so large a proportion of nitrogen as caffeine, its percentage of this element amounting to 21.6. Its composition, as shown by Liebig, is represented by the formula C8H10N4O2, by which it appears to be closely related to some of the nitrogenized constituents of bile, as taurine, also to methyl-theobromine. It is obtained crystallized in long silky needles of a white color, which are fusible and volatile, and are easily dissolved in water, alcohol, and. ether. To a decoction of coffee or tea acetate of lead is added to precipitate the caffeotannic acid. This is separated from the solution by filtering, and the excess of lead is removed by its precipitation by sulphuretted hydrogen. The liquor, again filtered, is then evaporated, and the caffeine crystallizes. It is purified by dissolving and again crystallizing.

The quantity obtained from coffee is generally about 1 per cent., which is only one half the amount furnished by tea; as the infusions are prepared, however, for ordinary use, more of the active principle is contained in a cup of coffee than in one of tea. Robiquet and Boutron give much larger proportions than 1 per cent. In Java coffee they found 4.4 per cent, of caffeine, and in Martinique coffee 6.4 per cent. Caffeine has a bitter taste, and acts powerfully upon the system when taken in doses of from 2 to 10 grains. It causes palpitation of the heart, great irregularity of the pulse, oppressions in the chest and pains in the head, confusion of the senses of hearing and seeing, sleeplessness, and delirium. The substance may be recognized by its great volatility, and the property it possesses, when dissolved in nitric acid, evaporated to dryness, and exposed to ammoniacal gas, of giving a pink-colored blush. Caffeine evaporated to dryness with a little chlorine water yields a purple-red residue, which becomes golden yellow when more strongly heated, but red again on addition of ammonia; by this reaction it may be detected even in a single coffee bean. - Caffeine has been formed synthetically by treating the silver compound of theobromine with methyl-iodide. On a large scale caffeine can be prepared from tea by heating the concentrated aqueous decoction with litharge, evaporating the decanted liquid to a sirup, and treating it with pearlash and alcohol.

Claus finds that the inferior commercial sorts of tea are richer in caffeine than the finer and more fragrant varieties. - Caffeine is sometimes used in medicine to produce sleep, allay nervous irritability, and relieve sick headache. It should only be used for these purposes in cases of debility. Inflammation con-traindicates its use. The dose is one or two grains repeated every hour till eight or ten grains are taken if necessary.