The compound ſalts hitherto enumerated are ſuch as are compoſed of two ingredients only; but ſometimes three or more are ſo united as not to be ſeparated by chryſtallization. The vitriols that we are acquainted with are hardly ever pure, and two or three of them ſometimes are joined together.

Sometimes likewiſe it happens that neutral ſalts join earthy ſalts, and earthy ſalts metallic ones. I generally diſtinguiſh compound ſalts according to the number of their principles, whether the ſame acid be joined to ſeveral baſes, or the ſame baſis to different acids; or laſtly, whether ſeveral menſtrua and ſeveral baſes are joined together. Hence ariſe ſalts triple, quadruple, &c. which the diligence of after times muſt illuſtrate. I ſubjoin the moſt remarkable examples of triple and quadruple native ſalts which have occurred to me.

ALKALI MINERALE Salitum (common ſalt) contaminated by magneſia ſalita. The common ſalt when pure does not deliqueſce, but this degree of purity is ſeldom found, and in the native foſſil production (ſal gem) never.

ARGILLA vitriolata (alum) native, contaminated by vitriol of iron. In the aluminous ſchiſtus it ſometimes effloreſces in a feathery form. Is this the plumoſe alum of the antients?

ARGILLA vitriolata (alum) native; contaminated by ſulphur and vitriolic acid.

At the places about Wedneſbury and Bilſton, in Staffordſhire, where the coal pits are on fire, this ſubſtance ſublimes to the ſurface, and may be collected in conſiderable quantity during dry or froſty weather. I cannot be certain that this is a true chemical union, but the eye cannot diſtinguiſh the parts. Perhaps the ſulphur volatilizes the alum and ſo becomes intimately mixed with it. The exceſs of vitriolic acid keeps it in a deliqueſcent ſtate.

I believe a ſimilar compound ſubſtance ſublimes at the Solfaterra near Naples. W.

ARGILLIA vitriolata (alum) native, contaminated by vitriol of cobalt. In the mines of Herregrund and Idra this may be ſeen, ſhooting out into long ſlender filaments. Perhaps this is the trichites of the Greeks. Diſſolved in water it immediately betrays the preſence of vitriolic acid, upon the addition of terra ponderoſa ſalita (muriatic acid ſaturated with heavy earth.) By the addition of phlogiſticated alkali a precipitate of cobalt is thrown down, which makes a blue glaſs with borax or microcoſmic ſalt.

CUPRUM vitriolatum (vitriol of copper) contaminated by iron.

FERRUM vitriolatum (vitriol of iron) contaminated by nickel.

CUPRUM vitriolatum (vitriol of copper) and vitriol of iron contaminated by zinc. Such is found at Fahlune.