Tea contains three active principles to which it owes its peculiar properties and characteristics. These are theine, a crystallisable alkaloid, to which is due its stimulating and restorative properties: tannin, whence it derives its astringency and a good deal of what is popularly known as "strength"; and a volatile oil, to which it owes its aroma. There is probably also a bitter principle, which has not yet been separated; it is less soluble than tannin, and is extracted from the leaves and passes into the water after a long infusion or stewing. Both theine and tannin are soluble in boiling water. The theine is in combination with tannic acid, and the theine and tannin are together dissolved out of the leaves into the water. There is a popular impression that by a very short infusion of only two or three minutes, theine can be obtained from tea, without the tannin. This is a mistake. Tannin, which is very soluble, is always dissolved out with the theine. Some teas, however, continue to give off tannin after the chief part of the theine has been dissolved out. These are chiefly the Indian teas, which have been highly fired and much fermented in the process of preparation.

The following table, taken from a number of analyses recently made by the late Professor Dittmar, shows clearly the effect of allowing the water to stand on the tea leaves five minutes and ten minutes respectively, and the varying amount of theine and tannin given off by China, Ceylon, and Indian teas: -

From this it will be seen that though Indian tea contains 25 per cent. more theine than China tea, it also contains 100 per cent. more tannin. Indian teas are much more widely used in England than China teas, and the "strong syrupy teas," advertised as of good value, and so largely consumed by the working classes, are, as a rule, blends of various Indian teas rich in tannin and astringent matters. It thus obviously becomes a matter of great, and even of national importance, considering how extensively and continuously tea is drunk, to ascertain the physiological effects of its principal constituents - theine and tannin.