To obtain this as pure as poſſible, the ſpathum ponderoſum § 89 (heavy ſpar) muſt be reduced to a fine powder, and with equal parts of fixed alkaly and charcoal duſt roaſted for an hour in a covered crucible. Powder the maſs, and add nitrous or muriatic acid diluted until all efferveſcence ceaſes, and the liquor be ſenſibly acid. To this liquor add mild fixed alkaly, and the heavy earth will be precipitated in a mild ſtate. If the acids or the alkaline ſalt contain any vitriolic acid, the heavy ſpar will immediately be regenerated. What remains undiſſolved by the acid is heavy ſpar, not decompoſed. The proceſs may be repeated upon this, but the product will then contain ſome martial earth and ſome clay from the crucible, therefore the firſt part will be the moſt pure.
TERRA PONDEROSA aerata, (heavy earth) has a ſpecific gravity of 3, 773[38]. 100 parts of it contain about 28 of water, 7 of aerial acid, and 65 of pure earth. It efferveſces with acids: with the vitriolic acid forms heavy ſpar, not ſoluble in water; with the nitrous and muriatic acids, it yields chryſtals, not very readily ſoluble; but with the vegetable acid the chryſtals deliqueſce.
When free from all contamination of acid or alkaly it ſcarcely melts in the fire, but loſes ³⁵⁄₁₀₀ of its weight. When united with the matter of heat, (i. e. rendered cauſtic) it diſſolves in 900 times its weight of water; and when this ſolution is expoſed to the atmoſphere, a cream or cruſt ſeparates at the top, which efferveſces with acids. After burning, it unites to acids without efferveſcence; but heat is produced, and the union is more tardy than when it is in a mild ſtate[39].
When cauſtic, it expels the volatile alkaly from ſal ammoniac, and forms a hepar with ſulphur, the watery ſolution of which is but imperfectly decompoſed by the nitrous or muriatic acids, upon account of the remarkable attraction betwixt this earth and the acid of the ſulphur, which it even takes from the vegetable alkaly[40].
When we compare theſe properties with thoſe which belong to common calcareous earth, mentioned at (§§ 92, 93), we ſhall readily ſee wherein they agree, and wherein they differ.
TERRA PONDEROSA vitriolata (heavy ſpar) is full four times as heavy as an equal bulk of water. It diſſolves entirely, though ſparingly, in concentrated boiling vitriolic acid, but the addition of a ſingle drop of water occaſions a precipitation. The ſame thing happens to gypſum; but that requires much leſs acid to diſſolve it, and the precipitation is made more ſlowly. If the heavy ſpar contained any ſulphur, it muſt certainly have appeared when the whole was diſſolved, but I never could find any thing like it.
Cronstedt, Min. § 18. 2.
Marmor metallicum druſicu § 19 C. Ponderous Spar.
TERRA PONDEROSA vitriolata, impregnated with bitumen, and mixed with gypſum, alum, and ſiliceous earth.
Cronstedt Min. § 24. Lapis hepaticus. Liver Stone.
A nucleus of this kind, taken out of a piece of alum ore from Andrarum in the province of Skone, yielded, in 100 parts, by analyſis, 33 of ſiliceous earth, 29 of cauſtic heavy earth, earth of alum about 5, and quick-lime from 3 to 7, beſides the water and vitriolic acid. By calculation it appears, that theſe baſes, together with vitriolic acid enough to ſaturate them, ought to weigh 71, which, with the addition of 33, exceeds the amount of the original 100. This increaſe points out the difference of a maſs newly chryſtallized, and of one carefully dried.
When we conſider that the terra ponderoſa was altogether unknown before the year 1774, and that many mineralogiſts are even now unacquainted with it, we cannot wonder that we know ſo few ſpecies of it. I have ſcarce a doubt but the terra ponderoſa aerata may be found mixed with other earths in many ſpecimens, when they come to be examined by chemical means more accurately than they could be heretofore. (See notes to §§ 58 and 88.)