Volatile alkali is procured by burning animal substances; in Egypt (from whence, as contained in _sal ammoniac_, we till of late imported it) from camel's dung; but now from bones, by distillation. To the liquor thus procured they add vitriolic acid, or substances which contain it. This acid unites with the alkali, and common salt being put to it, a double affinity takes place. The vitriolic acid uniting with the mineral alkali of the salt, makes _Glauber salt_, and the marine acid uniting with the volatile alkali, makes _sal ammoniac_. Slaked lime added to this, unites with the marine acid of the ammoniac, and sets loose the volatile alkali in the form of _alkaline air_, which combining with water, makes the liquid caustic volatile alkali. If chalk (containing calcareous earth united with fixed air) be mixed with the sal ammoniac, heat will make the calcareous earth unite with the marine acid, while the fixed air of the chalk will unite with the volatile alkali, and assume a solid form, being the _sal volatile_ of the apothecaries. LECTURE XVII. _Of Liquid Inflammable Substances._ Of liquid inflammable substances the principal is _spirit of wine_, sometimes called _ardent spirit_, and, when highly rectified, _alcohol_. It is obtained from vegetable substances by their going through the vinous fermentation. It is considerably lighter than water, colourless, and transparent, has a peculiar smell and taste, and the property of inebriating. Ardent spirit seems to consist of a peculiar combination of phlogiston and water; for when the vapour of it is made to pass through a red-hot earthen tube, it is resolved into water and inflammable air. It is highly inflammable, and burns without smoke, or leaving any residuum; and in the act of burning its phlogiston so unites with dephlogisticated air as to make fixed air. Ardent spirit mixes readily with water in all proportions, and also with essential oils, and balsams or resins, which are the same thing inspissated. By its affinity with essential oils, ardent spirit extracts them froth aromatic plants; and these liquors have obtained the name of _tinctures_. When the tinctures are distilled, the more volatile parts of the essential oils, which come over in distillation, have acquired the name of _waters_; as _Lavender water_, _Rosemary water_, &c. and what remains in the still is called the _extract_ of the plant. If the tinctures be diluted with much water, the resinous part of the plant will be obtained pure, and separated from the extractive part, which will remain dissolved in the water, while the resin separates from it.