That the most solid of these, as amber, has been formerly in a liquid state, is evident, from insects and other substances being frequently found in them; and pit-coal has been often found with both the internal texture and external appearance of wood; so that strata of pit-coal have probably been beds of peat in some former state of the earth. LECTURE XIX. _Of Solid Substances._ All solid substances are capable of becoming fluid by heat, and most of them may thereby be reduced into a state of vapour, or air; and in passing from a fluid into a solid state their component parts assume a particular mode of arrangement, called _crystallization_, which differs according to the nature of the substance; so that all solids, especially if they be suffered to concrete slowly, may be called _crystals_. Exclusive of _salts_, which have been considered already, as formed by the union of acids and alkalis, solids in general have obtained the names of _earths_, or _stones_, which differ only in their texture; and they are distinguished into those that are _metallizable_, or those that are not; the former being called _ores_, and the latter simply _earths_; the principal of which are the _calcareous_, _siliceous_, _argillaceous_, _magnesia_, _terra ponderosa_, and a few others which have been discovered lately, but have not been much examined. _Of Calcareous Earth._ Calcareous earth is found in the shells of fishes, the bones of animals, chalk, lime-stone, marble, and gypsum: but all calcareous earth is supposed to be of animal origin; and beds of chalk, lime-stone, or marble, are thought to have been beds of shells formed in the sea, in some pristine state of the earth. The calcareous earth which is found in shells, lime-stone, and marble, is combined with fixed air, discovered by effervescing with acids. To obtain it perfectly pure, the earth must be pounded and washed with water, in order to free it from any saline substance which may be contained in it, then dissolved in distilled vinegar, and precipitated by mild alkalies. Lime-stone exposed to heat loses about half its weight, in fixed air and water, and the remainder, called _quick-lime_, attracts water very powerfully, and their union is attended with much heat, after which it dissolves into a fine powder called _slaked lime_. If it be left exposed to the atmosphere, it will of itself, by gradually imbibing moisture, fall into the state of powder. Water dissolves about one seven hundredth part of its weight of quick-lime, and is then called _lime-water_. Exposed to the air, a crust will be formed on its surface, which is found to consist of calcareous earth and fixed air. Lime and water mixed with sand make _mortar_, by which means different stones may be made to cohere as one mass, which is the most valuable use of this kind of earth.