Although theſe different mixtures are poſſible, and probably do exiſt, they have not yet been all found. The natural compoſitions of acids with the earths, forming ſubſtances not ſoluble in 1000 times their weight of boiling water, and which may be called ſaline earths, muſt be added to the ſpecies, as they are certainly chemical combinations.
The primitive earths hitherto detected are,
| TERRA PONDEROSA, or | heavy earth. |
| CALX, | calcareous earth. |
| MAGNESIA, | magneſia. |
| ARGILLA, | argillaceous earth. |
| TERRA SILICEA, | ſiliceous earth. |
And we muſt believe theſe to be primitive, until it ſhall appear by proper experiments that they may be ſeparated into others ſtill more ſimple, or changed into one another by art.
Theſe are firſt to be conſidered in their greateſt ſimplicity and purity, although nature never preſents us with ſuch, nor can they even by art be rendered abſolutely free from all heterogeneous mixture. Water and aerial acid readily unite with the four firſt, and when expelled by fire, a little of the matter of heat is added, and remains until driven out by a more powerful attraction. But in this ſtate they poſſeſs a degree of purity not to be attained by any other known method. Therefore it is neceſſary to examine them when ſufficiently burnt in order to diſtinguiſh better what properties depend upon adhering heterogeneous matters.
HEAVY EARTH,
OR
TERRA PONDEROSA.
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.