Part 12

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.

§ 112.

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.

§ 113.

ARGILLA (argillaceous earth) united to ſiliceous matter only.

CRONSTEDT Min. §78. Argilla porcellana. Porcelain clay.

Pipe clay.

I never examined any clay which did not contain a large quantity of ſiliceous earth; generally more than half its weight.

§ 114.

ARGILLA (argillaceous earth) united to ſiliceous and irony matter.

CRONSTEDT Min. §§ 87 and 90. Bole. Dye-earth.

Clay.

§ 115.

ARGILLA (argillaceous earth) united to ſiliceous and calcareous matter.

CRONSTEDT Min. §25. Marga argillacea. Marle.

§ 116.

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.

§ 116*.

ARGILLA (argillaceous earth) united to ſiliceous, calcareous, and magneſia earths.

Lithomarga.() CRONSTEDT Min. § 84. A.

Stone marrow.

§ 117.

ARGILLA (argillaceous earth) contaminated by vegetable alkaly and ſulphur, or at leaſt by the acid of ſulphur.