By earth of alum (argilla) I do not mean common clay, which is never free from ſiliceous matter, but a pure clay, unmixed, at leaſt, with any other earth. It may be readily obtained by diſſolving Roman or roach alum in diſtilled water, filtering, and precipitating by mild volatile alkaly.
The ſpecific gravity of this pure clay, or earth of alum, is 1,305. It diſſolves in acids, with a little efferveſcence. With the vitriolic acid it forms alum; with the nitrous, muriatic and vegetable acids, deliqueſcent ſalts.
When dry, it abſorbs water greedily, becomes ſoft, and, with a due quantity of water, gains ſuch a tenacity, that it may be moulded at pleaſure. This maſs contracts greatly in the fire, from whence ariſe numerous cracks; and with a due degree of heat, it becomes hard enough to ſtrike fire with ſteel. By this burning it loſes its glutinous tenacity, and the water is excluded by the approach of the particles; nor does it again aſſume its former properties, but by ſolution and precipitation.
It may be diſſolved in the dry way, by means of fixed alkaline ſalt, as well as in the liquid way, by acids. The vitriolic acid is better than the others for this purpoſe, becauſe more eaſily concentrated.
Earth of alum neither diſſolves ſulphur, nor decompoſes ſal ammoniac.
ARGILLA (argillaceous earth) united to ſiliceous matter only.
Cronstedt Min. §78. Argilla porcellana. Porcelain clay.
I never examined any clay which did not contain a large quantity of ſiliceous earth; generally more than half its weight[44].
ARGILLA (argillaceous earth) united to ſiliceous and irony matter.
Cronstedt Min. §§ 87 and 90. Bole. Dye-earth.
ARGILLA (argillaceous earth) united to ſiliceous and calcareous matter.
Cronstedt Min. §25. Marga argillacea. Marle.
ARGILLA (argillaceous earth) united to ſiliceous earth and magneſia.
Cronstedt Min. §§ 84, 4. B. Terra lemnia.
Its component parts reſemble thoſe of talc, but differ in their proportions, and are alſo leſs intimately united.
ARGILLA (argillaceous earth) united to ſiliceous, calcareous, and magneſia earths.
Lithomarga.([45]) Cronstedt Min. § 84. A.
ARGILLA (argillaceous earth) contaminated by vegetable alkaly and ſulphur, or at leaſt by the acid of ſulphur.
Cronstedt Min. § 124. 2. b. Minera aluminis romani.
It certainly contains vitriolic acid[46], and perhaps, alſo, a ſmall portion of ſulphur. The vegetable alkaly ſufficiently ſhews its volcanic origin.
ARGILLA (argillaceous earth) contaminated by ſiliceous matter, pyrites, and petroleum.
Cronstedt Min. § 124. 2. c. Schiſtus aluminaris[47].
ARGILLA (argillaceous earth) intimately united with leſs than half its weight of ſiliceous earth, and a ſmall quantity of mild calcareous earth.
Cronſtedt Min. §§ 43–48. Gemma.
The Gems ſuffer no change under the blowpipe, with foſſil fixed alkaly, but are diſſolved by microcoſmic ſalt and borax.
| To this head belong | Rubinus, the | ruby; |
| Saphirus, | ſapphire; | |
| Topazius, | topaz; | |
| Smaragdus, | emerald. |
The tourmaline holds a kind of middle place betwixt the gems and the ſcherle. The colour, in all of them, is owing to iron.
ARGILLA (argillaceous earth) intimately united to half its weight of ſiliceous earth (or more), and a little mild calcareous earth. Scherle.
Cronſtedt Min. §§ 68–71. Granatus et Baſaltes, which I call Scherle.
The remote varieties of theſe are eaſily diſtinguiſhed, the near ones difficultly.
ARGILLA (argillaceous earth) looſely united to half its weight, or more, of ſiliceous earth, and a little calcareous earth.
Cronstedt Min. §§ 108–112. Zeolithus. Zeolite.
There is a great affinity betwixt this and Scherle; but in the zeolite, the component parts cohere ſo looſely, that acids attach and ſeparate them without their being previouſly treated with alkalies; but this is not the caſe with the ſcherles.
Zeolite, contaminated by magneſia, I have not yet examined.
ARGILLA (argillaceous earth) intimately united to a large proportion of ſiliceous earth, and a ſmall proportion of magneſia.
Cronstedt Min. §§ 93–96. Mica. Talcum. [48]Glimmer. Talc.