_Limonite Iron._--Fusibility about 5; yellow and reddish beads; water given off in closed tube; in reducing flame magnetic residue on charcoal; soluble in hydrochloric acid after a short time. _Iron Pyrites._--Fusibility about 2; yellow and red beads; in closed tube yellow precipitate due to sulphur; magnetic after reduction on charcoal; insoluble in hydrochloric acid. _Rock Salt._--Intense yellow sodium flame; fusibility about 1; microcosmic salt with copper oxide shows strong chlorine reaction--a fine blue flame surrounding the bead when re-introduced into the flame. It is soluble in water. _Selenite_ (_Gypsum_).--Fusibility about 2.5; brilliant flame; in closed tube it becomes white and opaque and much water is given off; with soda, on charcoal, sulphur reactions are obtained; soluble in hydrochloric acid. _Calcite_ (_Carbonate of Lime_).--Flame glows very strongly; infusible; effervesces freely in cold hydrochloric acid. _Dolomite._--Flame, with hydrochloric acid, like calcite; infusible; effervesces in hot hydrochloric acid. _Magnesite._--Infusible; with cobalt nitrate a fair magnesia reaction on charcoal, _i.e._, turns into a dull pink; effervesces in hot hydrochloric acid. _Manganese._--With borax in oxidising flame a red-violet bead is obtained, but with the reducing flame it is colourless. The above are commonly met with in brick-earths; for other minerals and substances also found, the reader may be referred to special works dealing with blowpipe analysis. CHAPTER VIII. THE CHEMISTRY OF BRICK-EARTHS (_Continued_). In this chapter we shall fulfil our promise (_ante_ p. 58) to explain in an elementary manner the precise meaning of ordinary commercial chemical analyses of some typical earths used in brickmaking, etc. We may commence by explaining a few terms used by the chemist. An _atom_ is the smallest imaginable portion of matter, and all matter is said to consist of atoms. A _molecule_ is the smallest conceivable combination of atoms, and every compound substance is ultimately built up of molecules. An _element_ is a substance that has hitherto defied the efforts of the chemist to subdivide or split up. Over seventy of these elementary substances are at present known, and their number is being constantly added to. Again, by improvement in analytical methods, a so-called element may be subdivided, and thus removed from the list. The elements are classified into _metals_ and _non-metals_; and it is convenient to give each of them a symbol to save trouble in writing, and to render clearer to the reader the chemical nature of a compound body. Thus, the symbol for the element aluminium is Al; for silicon Si; for carbon C; for calcium Ca; for oxygen O; for iron Fe; for hydrogen H; for chlorine Cl; and so on.