The condensing apparatus used with the common still is very simple. The tube in which the head terminates is in serted into the upper end of a pipe, which is kept cool by passing through a vessel filled with water, called the refrigeratory. This pipe is commonly made of a serpen tine form ; but as this renders it difficult to be cleaned, Dr. Black recommends a sigmoid pipe- The refrigeratory may be with a stopcock, that when the water it contains becomes too hot, and does not condense all the vapour produc ed, it may be changed for cold water. From the lower end of the pipe, the pro duct of the distillation drops into the ves sel destined to receive it ; and we may observe, that when any vapour issues along with it, we should either diminish the power of the fire, or change the wa ter in the refrigeratory. There was a process of this kind, called circulation. It consisted in arranging the apparatus, so that the vapours were no sooner condens ed into a fluid form, than this fluid- re turned back into the distilling vessels, to be again vaporised ; and was effected by distilling in a glass vessel, with so long a neck that the vapours were condensed be fore they escaped at the upper extremity, or by inverting one matrass within ano ther. When corrosive substances are distilled in this way, the cucurbit and alembic are used ; but these substances are more conveniently distilled per la 'us.
The distillation per lotus is performed in a retort, or pear-shaped vessel, having the neck bent to one side. The body of a good retort is well rounded, uniform in its appearance, and of an equal thickness, and the neck is sufficiently bent to allow the vapours, when condensed, to run freely away, but not so much as to ren der the application of the receiver incon venient, or to bring it too near the fur nace. The passage from the body into the neck must be perfectly free and suf ficiently wide, otherwise the vapours pro duced in the retort only circulate in its body without passing over into the re ceiver. For introducing liquors into the retort without soiling its neck, which •would injure the product, a bent funnel is necessary. It must be sufficiently long to introduce the liquor directly into the body of the retort ; and in withdrawing it, we must carefully keep it applied to the upper part of the retort, that the drop hanging from it may not touch the inside of the neck. In some cases, where a mixture of different substances is to be distilled, it is convenient and necessary to have the whole apparatus properly ad. before the mixture is made, and we must therefore employ a tubulated retort, or a retort furnished with an aperture, accurately closed with a ground stopper. This tubulature should be placed on the upper convex part of the retort before it bends to form the neck, so that a fluid poured through it may fall directly into the body without soiling the neck.
Retorts are made of various materials. Flint-glass is commonly used when the heat is not so great as to melt it. For distillations which require excessive de grees of heat, retorts, of earthenware, or coated glass retorts are employed. Quicksilver is distilled in iron retorts.
The simplest condensing apparatus used with the retort is the common glass receiver ; which is a vessel of a conical or globular form, having a neck sufficient ly wide to admit of the neck of the retort being introduced within it. To prevent
the loss and dissipation of the vapours to be condensed, the retort and receiver may be accurately ground to each other, or secured by some proper lute. To pre vent the receiver from being heated by the caloric evolved during the condensa. tion of vapours in it, we must employ some means to keep it cool. It is either immersed in cold water, or covered with snow, or pounded ice, or a constant evaporation is supported from its stir. face, by covering it with a cloth, which is kept moist by means of the descent of water, from a vessel placed above it, through minute syphons or spongy wor sted threads. But as, during the process of distillation, permanently elastic fluids are often produced, which would endan ger the beeaking of the vessels, these are permitted to escape either through a to. bulature, or hole, in the side of the re. ceiver, or rather through a hole made in the luting. Receivers having a spot is suing from their side, are used when we wish to keep separate the products ob tained at different periods of any distilla tion. For condensing very volatile vapours, a series of receivers, communicating with each other, termed adopters, were for merly used ; but these are now entirely superseded by Woulfe's apparatus, whicTt consists of a tubulated retort, adapted to a tubulated receiver. With the tubula ture of the receiver, a three-necked Mt, tle is connected by means of a bent tube, the further extremity of which is im. mersecl, one or more inches, in some fluid contained in the bottle. A series of two or three similar bottles are connected with this first bottle in the same way. In the middle tubulature of each bottle a glass tube is fixed, having its lower extremity immersed about a quarter of an inch in the fluid. The height of the tube above the surface of the fluid must be greater than the sum of the columns of fluid standing over the further extremities of the connecting tubes, in all the bottles or vessels more remote from the retort. Tubes so adjusted are termed tubes of safety, for they prevent that reflux of fluid from the more remote into the nearer bot tles, and into the receiver itself, which would otherwise inevitably happen on any condensation of vapour taking place in the retort, receiver, or nearer bottles. Different contrivances for the same pur. pose have been described by Messrs. Welter and Burkitt ; and a very inge nious mode of connecting the vessels without lute has been invented by Citi zen Girard, but they would not be easily understood without plates. The further tubulature of the last bottle is commonly connected with a pneumatic apparatus, by means of a bent tube. When the whole is properly adjusted, air blown into the retort should pass through the receiver, rise in bubbles through the fluids con tained in each of the bottles, and at last escape by the bent tube. In the re. ceiver, those products of distillation are collected which are condensable by cold, alone. The first bottle is commonly filled with water, and the others with alkaline solutions, or other active fluids ; and as the permanently elastic fluids produced are successively subjected to the action of all these, only those gases will escape by the bent tube which- are not absorbable by any of them.