As soon as the matters contained in the alembic grow hot, vapours begin to arise from them, and ascending through the neck of the alembic into the head, are by the sides thereof stopped and condensed: from thence they trickle down in little streams to the groove, which conveys them to the spout; and by that they pass out of the alembic into a glass vessel with a long neck, the end of the spout being introduced into that neck, and luted thereto. To facilitate the refrigeration and condensation of the vapours circulating in the head, all alembics of metal are moreover provided with another piece, which is a kind of large pan of the same metal, fitted and soldered round the head. This piece serves to keep cold water in, which incessantly cools the head, and therefore it is called the _Refrigeratory_. The water in the refrigeratory itself grows hot after some time, and must therefore be changed occasionally; the heated water being first drawn off by means of a cock fixed near the bottom of the refrigeratory. All copper alembics should be tinned on the inside for the reasons already given. When saline spirits are to be distilled, alembics of metal must not be used; because the saline vapours would corrode them. In this case recourse must be had to alembics of glass. These consist of two pieces only; namely, a _Cucurbit_, whose superior orifice is admitted into and exactly luted with its _Head_, which is the second piece. In general, as alembics require that the vapours of the matter to be distilled should rise to a considerable heighth, they ought to be used only when the most volatile principles are to be drawn from bodies: and the lighter and more volatile the substances to be separated by distillation are, the taller must the alembic be; because the most ponderous parts, being unable to rise above a certain heighth, fall back again into the cucurbit as soon as they arrive there, leaving the lighter to mount alone, whose volatility qualifies them to ascend into the head. When a matter is to be distilled, that requires a very tall alembic, and yet does not admit of a metalline vessel, the end will be best answered by a glass vessel of a round or oval shape, having a very long neck, with a small head fitted to its extremity. Such a vessel serves many purposes: it is sometimes employed as a receiver, and at other times as a digesting vessel; on which last occasion it goes under the name of a _Matrass_. When one of these, provided with a head, is applied to the purpose of distilling, it forms a sort of alembic.