Extraction being finished, the cock _o_ is opened, and then the cock _a_, and the fluid allowed to run into the flask of the distilling apparatus (Fig. 2). For working on a large scale, the flask is, however, too small, and is suitably replaced by a bottle-shaped tin vessel, _F_ (Fig. 11), the conical cover _D_ of which is secured by means of the rubber ring _R_ and iron screw-clamps, _S_. A bent glass tube fitted into the cover is connected with the cooling-pipe of the apparatus shown in Fig. 2. But the oils prepared by extraction are not sufficiently purified by mere rectification, as traces of the solvent adhere tenaciously to them, which can only be removed by passing a current of air through the oil. But contact with air has an injurious effect upon the delicacy of the odor. For expensive oils a current of air should therefore never be used, but one of pure carbonic acid. Fig. 12 shows a suitable apparatus for the purpose. The large bottle _A_, filled half full with pieces of white marble, is closed with a doubly-perforated cork; through one of the holes is inserted a funnel-tube, and through the other a short tube bent at a right angle. The latter is connected with another tube which reaches to the bottom of the vessel _B_, in which is also inserted a tube open in the bottom, and a short tube bent at a right angle. Alongside _B_ stands another vessel, _C_, arranged in the same manner. The tube leading from _C_ is connected with a tin pipe, _D_, with a rose-like expansion on its lower end. This pipe is inserted in the glass balloon containing the volatile oil. Finally, a pipe leads to the flask _F_, filled with water. [Illustration: FIG. 12.] To put the apparatus in operation, strongly diluted hydrochloric acid is poured through the funnel-tube upon the pieces of marble in _A_, which causes the development of a current of carbonic acid. But as the latter carries along water and hydrochloric acid, it has to be freed from them before coming in contact with the volatile oil. The vessels _B_ and _C_ serve for the purpose. _B_ is half filled with water, while _C_ contains strong sulphuric acid. In _B_ the hydrochloric acid carried along with the current of carbonic acid is retained, while the water is fixed on the sulphuric acid in _C_. The current of carbonic acid passing out from _C_ is perfectly pure, and enters the volatile oil through the fine perforations in the pipe _D_. It absorbs the traces of solvent still adhering to the oil, and finally passes out through the water in the bottle _F_. Volatile oils obtained by extraction, and purified by a current of carbonic acid, will keep for years without undergoing alteration, if placed immediately in hermetically closed vessels and stored in a dark place. Oils purified by a current of air always become somewhat thickly fluid by storing, and partially lose their fine odor, which is due to the oxygen absorbed during the process.