ALKALI VEGETABILE aeratum (mild vegetable alkaly) is hardly ever found native, unleſs in the neighbourhood of woods deſtroyed by fire.
In the year 1774, at Douai in Flanders, a ſpring was diſcovered ſurrounded by a wall, whoſe waters, beſides other impregnations, contained 11 grains of vegetable alkaly in a pint[[27]].
ALKALI MINERALE aeratum (mild foſſil alkaly, natron, the nitre of the ancients) is found plentifully in many places, particularly in Africa and Aſia, either concreted into chryſtallized ſtrata, or fallen to a powder; or effloreſcing on old brick walls, or laſtly, diſſolved in ſprings. It frequently originates from decompoſed common ſalt. I am not ignorant that the acid of common ſalt adheres ſtrongly to its baſis ſo as not to be expelled by fire; but perhaps the viciſſitudes of the atmoſphere continually acting for ages, may be more powerful. In immenſe plains covered over with this alkaly, ſcarcely any common ſalt is found upon the ſurface, but the deeper you dig the more it is contaminated by it, the common ſalt being yet undecompoſed for want of acceſs of air.
ALKALI VOLATILE aeratum (mild volatile alkaly) has been found in pump waters in London[[28]], in Lauchſtadt[[29]], at Frankfort on the Mayne[[30]], and copper immerſed therein is ſaid to have been diſſolved into a blue liquor.
The three alkalies mentioned above as ſaturated with aerial acid, differ greatly from cauſtic alkalies, in the mildneſs of their taſte, in their property of chryſtallizing, and in their efferveſcing with acids which expel the aerial acid, but they ſtill change vegetable blues to greens, though not ſo powerfully as the cauſtic alkalies do. Therefore, although the ſubtil aerial acid in other reſpects gives them neutral properties, yet in this it does it but imperfectly.
The compounds of earths and acids which poſſeſs ſolubility mentioned at § 20, are decompoſed and precipitated by mild, but not by phlogiſticated alkalies.