Separation of the oil and water.—As previously mentioned the specific gravity of most volatile oils is less than that of water. This behavior is utilized for the separation of the oil and water, by means of a so-called Florentine flask (Fig. 7). It consists of a glass flask provided near the bottom with a pipe, a, rising vertically to near the neck c of the flask where it is bent downwards as shown in the illustration. The mixed liquid of water and oil drips from the cooling pipe into the flask, and the water w, being specifically heavier, separates from the oil floating on the top, and gradually ascends in the pipe a, finally flowing over at d. Oils specifically heavier than water are caught in receivers provided with a discharge-pipe near the mouth of the flask as shown in Fig. 8.
The oil delivered from the receivers is, however, still mixed with some water, dirt, etc., and for their separation is allowed to stand quietly for some time. The final separation is effected either by simply pouring off the oil, especially if larger quantities have to be handled, or with the assistance of a separator-funnel (Fig. 9). This consists of the glass-funnel T secured to the stand G, and provided with a close-fitting lid, P. The fluid is poured into the funnel, the lid placed in position, and the whole allowed to rest until the water W is completely separated from the oil O. The oil is then separated from the last drops of water by carefully opening the faucet H.
Most volatile oils are obtained by distillation, but this method is not practicable for separating the odoriferous principle of many of the most sweet-scented and delicate flowers, partially because the flowers contain too little oil, and partially because the oil would lose in quality if obtained by distillation.
Fig. 9.
Extraction.—For obtaining the volatile oils by extraction various solvents such as ether, bisulphide of carbon, etc., may be employed. Carefully rectified petroleum-ether is very suitable for the purpose. It completely evaporates at about 122° F., and when sufficiently purified does not possess a disagreeable odor. The process of extraction is briefly as follows: The material to be extracted is treated in a digester with petroleum-ether or one of the above-named solvents. The solution is then drawn off and the solvent evaporated in a still. The recondensed solvent flows immediately back into the digester and further extracts the material contained therein. The operation is repeated until nothing soluble remains. In practice some difficulties are, however, connected with this process since, besides the volatile oils, resins, and coloring and extractive substances are dissolved, which have to be removed, as well as the last traces of the solvent, as otherwise the oil would acquire a foreign odor. Further the solvents mentioned are very volatile and inflammable, requiring the greatest precautions as regards fire. For these reasons the extraction process is not suitable for many purposes, and though at first great hopes were entertained in regard to it, its use is limited to substances with a large content of volatile oil.
Fig. 10.
For extraction on a small scale, the apparatus, Fig. 10, is a very suitable one. It is especially adapted for manufacturers of perfumery, who wish to extract fresh flowers. It consists of a cylindrical vessel, C, of tin plate, provided on the bottom with the stop-cock a and the pipe b. The lid D fits into a gutter, R, running around the edge of C, and is hermetically closed by water in R. The cylinder is filled with the vegetable substance to be extracted, and sufficient petroleum-ether or bisulphide of carbon to cover it, poured in. The lid is then adjusted, the gutter R filled with water and the apparatus allowed to stand quietly for forty minutes. To remove the fluid from the cylinder, the faucet o in the lid is first opened, and then the stop-cock a; the fluid escapes at b, and is caught in a well-closed vessel. The operation may be repeated once or twice, or the vegetable substance is pressed out by means of a wooden plate, and the apparatus filled anew. The faucet h serves for emptying the gutter R.