Though this is denominated a weaker acid than the nitrous, yet it will take silver, lead, or mercury, from their union with the nitrous acid. Upon this principle, a solution of these metals in the nitrous acid will readily discover whether any water contains the marine acid, the latter uniting with the metal dissolved in the former, and forming with it, if it be silver, a _luna cornea_; which being a substance insoluble in water, discovers itself by a cloudy appearance. The union of the marine acid with earths forms salts that easily deliquesce, but with the metals such as are capable of crystallization; and so also is that formed by the union of this acid to terra ponderosa. Neither this acid nor the nitrous will dissolve gold or platina; but a mixture of them, called _aqua regia_, will do it. The marine acid has a strong affinity to dephlogisticated air, and will take it from manganese and other substances; and with this union it becomes a different acid, called _dephlogisticated marine acid_, being water impregnated with dephlogisticated marine acid air, described above. LECTURE XIV. _Of the Vegetable Acids, and others of a less perfect nature._ The principal of the vegetable acids are the _acetous_ and the _tartareous_. The acetous acid is the produce of a peculiar fermentation of vegetable substances, succeeding the _vinous_, in which ardent spirit it is procured, and succeeded by the _putrefactive_, in which volatile alkali is generated. Thus wine is converted into vinegar. Crude vinegar, however, contains some ingredient from the vegetable substances from which it was procured: but distillation separates them, and makes the vinegar colourless; though some of the acid is lost in the process. The acetous acid is concentrated by frost, which does not affect the proper acid, but only the water with which it is united. It may likewise be concentrated by being first combined with alkalies, earths, or metals, and then dislodged by a stronger acid, or by mere heat. Thus the acetous acid, combined with vegetable alkali, forms a substance that is called the _foliated earth of tartar_; and it may be expelled from it by the vitriolic acid. When combined with copper it makes _verdigris_; and from this union heat alone will expel it in a concentrated state. The acetous acid thus concentrated is called _radical vinegar_. Still, however, it is weaker than any of the preceding mineral acids. Several vegetables, as lemons, sorrel, and unripe fruit, contain acids, ready formed by nature, mixed with some of the essential oil of the plants, which gives them their peculiar flavours. All these acids have peculiar properties; but it is not necessary to note them in this very general view of the subject. Like vinegar, these acids may be concentrated by frost, and also by a combination with other substances, and then expelled by a stronger acid.