Substances which abound much in Oil, being bruised and triturated with water for some time, afford a liquor of an opaque dead-white colour, like milk. This liquor consists of such juices as the water is capable of dissolving, together with a portion of the oil, which being naturally indissoluble in water, is only divided and dispersed in the liquor, the limpidity whereof is by that means destroyed. This sort of oily liquor, in which the oil is only divided, not dissolved, is called an _Emulsion_. The oily particles in Emulsions spontaneously separate from the water, when left at rest, and uniting into greater masses rise, on account of their lightness, to the surface of the liquor, which by that means recovers a degree of transparency. If vegetables abounding in essential oils and resins be digested in spirit of wine, the menstruum takes up these oily matters, as being capable of dissolving them; and they may afterwards be easily separated from it by the affusion of water. The water, with which spirit of wine has a greater affinity than with oily matters, separates them by this means from their solvent, agreeably to the common laws of affinities. Without the help of fire, scarce any thing, besides the substances already mentioned, can be obtained from a plant: but, by the means of distillation, we are enabled to analyse them more completely. In prosecuting this method of extracting from a plant the several principles of which it consists, the following order is to be observed. A plant being exposed to a very gentle heat, in a distilling vessel set in the _balneum mariæ_, yields a water which retains the perfect smell thereof. Some Chymists, and particularly the illustrious Boerhaave, have called this liquor the _Spiritus Rector_. The nature of this odoriferous part of plants is not yet thoroughly known; because it is so very volatile that it is difficult to subject it to the experiments necessary for discovering all its properties. If, instead of distilling the plant in the _balneum mariæ_, it be distilled over a naked fire, with the precaution of putting a certain quantity of water into the distilling vessel along with it, to prevent its suffering a greater heat than that of boiling water, all the essential oil contained in that plant will rise together with that water, and with the same degree of heat. On this occasion it must be observed, that no essential oil can be obtained from a plant after the _Spiritus Rector_ hath been drawn off; which gives ground to think that the volatility of these oils is owing to that spirit. The heat of boiling water is also sufficient to separate from vegetable matters the fat oils which they contain. That, however, is to be done by the way of decoction only, and not by distillation: because, though these oils will swim on water, yet they will not rise in vapours without a greater degree of heat.