This, like the other primitive earths, is ſeldom found pure. In order to have it ſo, reduce clear quartz chryſtals into powder; melt it with four times the weight of fixed alkaly; diſſolve the whole in water; precipitate by a large quantity of ſtrong acid; carefully waſh and dry the precipitate.
The acid muſt be uſed in a ſuperfluous quantity, that any other earths contained may be diſſolved.
The ſpecific gravity of this earth, is 1,975. The particles, when firſt precipitated, occupy, in water, at leaſt twelve times the ſpace that they do when dried; ſo that, when ſufficiently fine, they may remain ſuſpended therein; nay, when vehemently heated in a cloſe veſſel, they may be diſſolved. No acid, except that of fluor ſpar (§ 30) has any action upon this earth. Fixed alkalies unite with it in the liquid way, but in the dry way they ſeize it with great vehemence, and convert twice their weight of it into a permanent transparent glaſs. Such is its affinity to alkalies, that it imparts to clay, which is always loaded with it, the power of ſeparating ſome of the acid from nitre and common ſalt. When pure, it is refractory in the fire.
Although ſiliceous earth is not altogether ſimple, yet, in mineralogy, it muſt be conſidered as primitive, until deciſive experiments ſhew us from which of the preceding earths it is derived[49].
TERRA SILICEA (ſiliceous earth) united to very ſmall quantities of calcareous and argillaceous earth.
Cronstedt Min. § 51. Quartzum. Quartz.
TERRA SILICEA (ſiliceous earth) united to argillaceous earth.
Cronstedt Min. § 58. Calcedonius. Chalcedony.
And perhaps the Opal. The Hydrophanus is only a variety of theſe.
Whether the carnelian, and other ſiliceæ, of finer or coarſer texture, belong to this or the preceding ſpecies I cannot yet determine with certainty.
TERRA SILICEA (ſiliceous earth), united to an argillaceous and highly martial earth.
Cronstedt Min. §§ 64, 65. Jaſpis. Jaſper.
TERRA SILICEA (ſiliceous earth), loaded with martial earth.
Cronstedt Min. § 53.
This ſpecies is often called jaſper, but improperly, becauſe it contains no argillaceous earth.
TERRA SILICEA (ſiliceous earth), united to argillaceous and a ſmall quantity of calcareous earth.
Cronstedt Min. § 63. Petroſilex. Chert.
TERRA SILICEA (ſiliceous earth) united to argillaceous earth and a little magneſia.
Cronstedt Min. §66. Feldſpathum. Feld ſpat.
TERRA SILICEA (ſiliceous earth), united to magneſia, mild calcareous earth, fluor ſpar and alſo to the calxes of copper and iron. Chryſopraſius. I have not examined this, but inſert it upon the experiments of Mr. Achard.
To determine accurately the ſpecies of earths is the moſt difficult part of mineralogy, for innumerable analyſes yet remain to be made. But that which now ſeems intricate and obſcure will become plain and eaſy when experiments have been ſufficiently multiplied.