DISTILLATION, a chemical rproce* which consists in separating bodies which are volatile from those which are more fixed, by the application of heat. All bo dies which are susceptible of the elastic or vaporous form, at the same time that they are not decomposed, or otherwise changed in their properties, are capable of being separated from other matter by distillation.
The process employed for distilling liquid bodies from other matter is sim ply called distillation ; that, on the con trary, used to separate solid bodies, by giving them the elastic form, is termed sublimation.
The apparatus employed for the first process are of several varieties, suited to the nature of volatile body. That employed for the distillation of water, alcohol, and the various essential oils, is called a still. It is chiefly made of cop per, and ought to be so constructed, that as great an evaporable surface as possi ble may be exposed. Those employed in the Scotch distilleries are in the form of an erect cone, the base of which is about four times the altitude. The va pour which comes from the still is con densed in two ways. The first is the oldest and most simple method, but not the most effective. The head, or capital of the still, is so formed, as to have a ves sel or cavity on the outside, containing cold water ; the inside is in the form of a cone, and sometimes of a dome, round the base of which is placed a channel, terminating in an inclined beak, or tube, to convey the liquid arising from the con densed vapour. When the vapour rises up against the conical, or dome-like sur face, the external water causes it in stantly to condense, and the drops run down the surface into the channel, from whence they pass into the delivering tube, and thence into the receiver. The other method of condensation consists in letting the vapour pass through a spi ral tube, fitted into the inside of a tub, which is filled with cold water, and so contrived that the product of the distil lation shall have no communication'with the external water. This tub, which is called a worm tub, should be supplied with cold water at the bottom, while the warm water, caused by the condensation of the vapour, should be made to run off at the top.
The apparatus employed for distilling bodies more easily condensible consist of two carts, one called the retort, con taining the substance to be distilled, and the other the receiver, because it receives the substance raised from the retort. In
the distillation of bodies which afford permanent gases as well as condensible matters, in addition to the receiver, a number of connected vessels are employ ed, constituting what, from its inventor, is called a Woulfe's apparatus, in which, what is not condensed, or absorbed in the first bottle, passes forward to the se cond, and so on to the third and fourth, till at length the absolutely incondens able part is received into a vessel called a gasometer. Before the invention of this apparatus, this kind of distillation was attended with great danger, both to the apparatus and the operator, the re ceiver being very liable to burst, and the fumes being intolerably suffocating. Both these inconveniencies are completely ob viated by the invention of the Woulfe's apparatus. Sometimes an apparatus call ed an alembic is used for distillation ; it is generally of glass, and is used for ex periments in the small way ; it consists of a bottle holding the substance to be dis, tilled, having a dome-like head, furnish, ed with a channel similar to that of the common stile ; indeed, it differs only from it in not having cold water on the out side. There are various modes of apply ing heat in distillation, depending upon the nature of the apparatus employed, as well as upon the substance to be distilled. The common still, which, being of metal, is immediately exposed to the naked fire, since from its tenacity, add its property of conducting heat with facility, it is not liable to crack, which is not the case with glass and earthen ware. If the ves sel holding the substance to be distilled be of the latter kind, it is essential to ap ply the heat very gradually and ly, and after the process, to suffer it to cool in a similar manner. This is effect ed in different ways; the most common of which is the sand bath, whiIt consists of a vessel of iron filled with fine dry sand. The retort, or other vessel, is imbedded in the sand previous to the application of the fire ; the inferior conducting power of the sand does not allow the heat to ap proach the retort, bud in that gradual way which will insure its safety from cracking. The heat must also be more uniform, since any sudden increase, or diminution of the fire, will not so imme diately affect the retort.