Part 11

CRONSTEDT Min. §30. See alſo §203. Hæmatites.

103.

There can be no doubt that the four firſt (§§ 94–97.), if not the laſt (§ 102), are genuine and diſtinct ſpecies; there is ſome difficulty as to the reſt, dependent, perhaps, only upon mechanical mixtures. If the heterogeneous matters can be diſcerned by the eye, we cannot heſitate to refer the ſubſtance to the ſaxa (ſtones); but in theſe the eye cannot diſcern them. Moreover, we know that the earths have a mutual attraction to each other, and form combinations more intimate than mechanical ones. Earth of alum, precipitated by a cauſtic alkali, and thrown into lime water, preſently loſes its pellucid and ſpongy texture, turns white, and condenſes, abſorbing the lime from the water, and forming an union not to be ſeparated but by chemical means.

From theſe conſiderations, I dare not venture to exclude doubtful ſpecies.

We ſay a thing is contaminated by another, when the mixture is of the mechanical kind; but when things are joined by the ſtronger power of attraction, we ſay they are united.

MAGNESIA.

§ 104.

Magnesia, called in the diſpenſatories, and by apothecaries magneſia alba, is a precipitation from its union with vitriolic acid, called Epſom ſalt. If this earthy precipitate be wanted in the greatest degree of purity, the Epſom ſalt must be taken chryſtallized, and well depurated, diſſolved in distilled water, and precipitated by volatile alkaly. Let the liquor be boiled for a few minutes, in order that what is kept in ſolution by the aerial acid may ſubside.

§ 105.

Magneſia, thus obtained, has a ſpecific gravity of 2,155. 100 parts of it contain about 25 of aerial acid, 30 of water, and 45 of earth. It diſſolves in acids, with a violent efferveſcence, but without heat. It again forms Epſom ſalt, with the vitriolic acid; with the nitrous acid it chryſtallizes, but the chryſtals are deliqueſcent; with the muriatic and vegetable acids it does not chryſtallize, and after drying, greedily attracts moiſture from the atmoſphere.

It does not melt in a moderate heat, but loſes ⁵⁵⁄₁₀₀ of its weight, and then has no attraction for water; diſſolves ſlowly, even in acids, and that without efferveſcence, but with ſome degree of heat. After calcination, it expels the volatile alkaly from ſal ammoniac, and unites to ſulphur, though very feebly.

MAGNESIA aerata (common magneſia) is never found native and unconnected, unleſs in waters, when it is diſſolved by an exceſs of aerial acid. (§ 66.)

§ 106.

MAGNESIA aerata (common magneſia) united with ſiliceous matter. This efferveſces with acids, and not unfrequently ſtrikes fire with ſteel.

§ 107.

MAGNESIA intimately united with ſiliceous matter. The ſoluble part is ſlowly taken up by acids, without efferveſcence.

CRONSTEDT Min. §§79–83. and perhaps § 102–105 alſo; but I have not yet ſubmitted the aſbeſti to the liquid analyſis.

Soaprock. Serpentine.

§ 108.

MAGNESIA united to argillaceous, ſiliceous, and pyritical matters.

M. MONNET diſcovered this, and the next ſpecies.

§ 109.

MAGNESIA united to argillaceous, ſiliceous, and pyritical matters, and likewiſe contaminated by petroleum.

This ſpecies reſembles aluminous ſchiſtus, but upon examination is found to contain more magneſia than clay.

§ 110.

All the ſpecies, except the firſt, are more or leſs contaminated by iron, but they do not owe all their colour to this ſubſtance. The green colours altogether vaniſh during ignition, and leave only a white opake maſs.

ARGILLACEOUS EARTH, OR ARGILLA.

§ 111.