Calcareous earth, united with vitriolic acid, makes _gypsum_; and this substance pounded and exposed to heat, parts with its water, and is then called _plaister of Paris_. In this state, by imbibing water again, it becomes a firm substance, and thus is useful in making moulds, &c. The earth of animal bones is calcareous united to the phosphoric acid. _Of Siliceous Earth._ Siliceous earth seems to be formed by nature from chalk, perhaps by the introduction of some unknown acid, which the vitriolic acid is not able to dislodge. It abounds in most substances which are hard enough to strike fire with steel, as _flint_, _rock crystal_, and most _precious stones_. It is not acted upon by any acid except the fluor and phosphoric, but especially the former: but it is soluble in alkalies; and being then dissolved in water, makes _liquor silicum_, from which the purest siliceous earth may be precipitated by acids. For this purpose about four times the weight of alkali must be made use of. With about equal weights of alkali and siliceous sand is made _glass_, of so great use in admitting light and excluding the weather from our houses, as well as for making various useful utensils. To make glass perfectly colourless, and at the same time more dense, commonly called _flint glass_, manufacturers use a certain proportion of calx of lead and manganese. Siliceous earth is not affected by the strongest heat, except by means of a burning lens, or dephlogisticated air. LECTURE XX. _Of Argillaceous Earth._ Argillaceous earth is found in _clay_, _schistus_, or _slate_, and in _mica_; but the purest is that which is precipitated from a solution of alum by alkalies; for alum consists of the union of vitriolic acid and argillaceous earth. This species of earth is ductile with water; it then hardens and contracts by heat, so as to be of the greatest use in forming _bricks_, or stones of any required form or size. By means of the property of clay to contract in the fire, Mr. Wedgwood has constructed an excellent thermometer to measure the degrees of extreme heat. The ductility of clay seems to depend upon some acid, probably the vitriolic, adhering to it; for it loses that property when it is burned into a brick, but recovers it when it has been again dissolved in an acid. _Of Terra Ponderosa._ _Terra ponderosa_, or _marmor metallicum_, is generally found in two states, viz. united to vitriolic acid, when it is called _calk_, or to fixed air, when it is called _terra ponderosa aerata_.