This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
I consider these two substances together, because their effects are of a closely similar character, and, where any difference exists, it can be readily indicated without producing confusion.
Coffee was introduced, under the officinal title of Caffea, into the U. S. Pharmacopoeia, at the late revision of that work, and fully merits the recognition.
Of the origin and physical properties of coffee and tea it is necessary to say little, as they are generally well known. It will be sufficient to state, in relation to coffee, that it is the seed of the Caffea Arabica, a small tree, indigenous in Arabia and Africa, and largely cultivated in various tropical countries of the old and new continents; and, in relation to tea, that it is the prepared leaves of at least two shrubs, distinct species of Thea, T. viridis, and T. Bohea, both natives of China, the former of which probably produces green, and the latter black tea.
As regards the composition of these two products, there is one principle which they both possess, and upon which it is highly probable that the effects they produce in common upon the system, in some measure at least, depend. This has been called caffein as found in coffee, and thein as in tea; but the name generally recognized at present is the former, from whatever source the principle may be derived. So far as we are at present concerned with caffein, it is sufficient to state that it is a highly nitrogenized body, with feeble basic properties, crystallizable, of a slightly bitter and disagreeable taste, and soluble in water, alcohol, and ether. A remarkable fact in connection with this principle is, that it has been found in three vegetable products, namely, coffee, tea, and Paraguay tea, all quite distinct in their botanical affinities, inhabiting widely distant regions of country, and yet each one of them employed by the natives of the several regions they inhabit, and for the same purposes, without any previous intercourse. Providence seems to have distributed them over the world for the enjoyment at least, if not for the profit of the human family; and a sort of instinct to have led to their discovery and use. Caffein is thought to exist in coffee partly free, and partly combined with a peculiar acid.
Besides this principle, which is common to coffee and tea, there is in the former, according to Pfaff, a variety of tannin which he calls caffeo-tannic acid; and, after it has been roasted for use, two new substances appear to have been generated, one a bitter principle, and the other an empyreumatic oil to which it owes its peculiar flavour in this state. It is probably to the combined influence of the caffein, the tannin, and the two newly developed empyreumatic products, that we must ascribe the effects of coffee on the system.
Tea probably owes its active properties chiefly to the caffein, which is in somewhat smaller proportion than in coffee, jointly with a considerable quantity of tannic acid of the variety found in galls, a peculiar bitter principle, and a volatile oil to which may be ascribed its characteristic aroma. Though the two varieties of tea, the green and black, differ considerably in their taste, they agree very closely in chemical composition, and in their effects on the system; the latter being somewhat weaker than the former.
For a more particular account of the origin, preparation, and sensible and chemical properties of coffee and tea, the reader is referred to the II. S. Dispensatory. I have here called attention only to those points which have some immediate bearing upon their physiological and therapeutic effects.