In all the preceding waters there is considerable carbonic acid. This acts as a sedative to the mucous membrane of the stomach, and is useful in indigestion. An aërated water added to milk renders it more digestible by diluting it, and by preventing the formation in the stomach of a heavy clot of casein. In the making of artificial aërated waters, it is essential that the water employed should be pure, that the acid used in generating the carbonic acid should be free from arsenic or other impurities, and that the water should not be allowed to come into contact with lead at any stage, as in pewter fittings. One per cent. of proof spirit is allowed in temperance beverages by the Excise.
TEA.
Tea is the leaf of an evergreen shrub, the Camellia thea, which is cultivated in China, Japan, British India, Ceylon, Java, and other countries. The tea leaves, as seen in this country, uncurl in hot water. They are lanceolated, with a serrated edge, and the veins do not extend to the edge of each leaf. By these characteristics they may be distinguished from foreign leaves, e.g., the sloe and willow used as adulterants (Fig. 4). The use of old and exhausted leaves can be detected by a determination of the percentage of soluble matter dissolved by boiling water from a given weight of tea. This on evaporation to dryness should be 28 to 30 per cent. of the total weight of the original tea. The presence of clay, iron dust or other forms of dust is detected by igniting a given amount of tea and determining the amount of ash. This should be only about six per cent.
In black tea, the leaves are dried in the sun, rolled and allowed to become soft and to ferment. During this process, some of the tannin appears to be converted into less soluble forms. The leaves are afterwards sun-dried, and these “fired” in a furnace. Green tea leaves are dried in the fresh condition over wood fires. Indian teas have more “body” and astringency than China teas. The smallest and topmost leaves of the tea plant give the finest sort of tea (Orange Pekoe); next to this comes Pekoe; the next largest leaves producing Souchong; after these Congou; while the coarser leaves nearer the base used to yield Bohea, which is now seldom seen.
Tea consists of three important constituents—volatile oil, theine or caffeine, and tannin—and soluble and insoluble extractive matters.
| The | amount | of | caffeine | varies | from | 2 to 4 | per | cent. |
| „ | „ | „ | tannin | „ | „ | 10 to 12 | „ | „ |
| „ | „ | „ | volatile | oil is about | ½ | „ | „ | |
(1) Volatile Oil gives the aroma and flavour to each particular tea. It is this which causes the headache, trembling, wakefulness, and restlessness, occasionally produced by tea, especially by green tea.
Leaves of
(A) Elder. (B) Tea. (C) Tea. (D) Sloe. (E) Elder.
Fig. 4.
(2) Theine or caffeine, is an alkaloidal crystalline principle. Its composition is represented by the formula C₈H10N₄O₂, H₂O. Ceylon tea, broken leaf contains 4·03 per cent., Assam (Indian) tea, broken leaf 4·02 per cent., while Chinese teas contain from 2·89 (Moyune Gunpowder) to 3·74 (Moning, black leaf) per cent. of caffeine (Allen).