the bark of the oak after it has been ground and used by the tanner. The smallest sort is generally made up in little square cakes called turf, and sold for firing. The coarser sort is sometimes dried in the sun, and used by bakers for heating their ovens, &c. but its chief use is for making hot-beds to raise pine-apples and other plants.—William III. introduced the use of it from Holland, for the purpose of raising orange trees; after which it was discontinued for many years: but about 1719, when ananas were first brought into England, it came into general use, and has ever since been in great estimation with gardeners for all the purposes of forcing, &c. on account of its strong and lasting fermentation. The smaller the tan the quicker it heats; but the larger sort acquires heat more gradually and retains it longer: the skilful gardener therefore uses the one or the other, or a mixture of both, according to the time and purpose for which it is wanted. It is some time after the tan comes out of the tanner's pit before it begins to heat, and therefore it is not fit for immediate use; but having lain a week or two, it enters into a state of fermentation, and if put into hot-beds properly prepared, will retain a moderate heat for three or four months. When it becomes useless for the hot-house, it is said by Miller and others to be an excellent manure for some kinds of land.

The word tan is sometimes, though improperly, used for the bark itself, which is the chief ingredient in the tanning of leather. Oak bark, on account of its great astringency and gummy-reinous properties, is preferred to all other substances for the purpose of tanning, as it not only preserves the leather from rotting, but also, by condensing the pores, renders it impervious to waters. See TANNING.

For an account of tan or tannin, considered as a chemical principle, see CHEMISTRY, No 2504.