It will be convenient to consider the behaviour of these two minerals in the kiln together, as the difference between them from that point of view is practically _nil_. Under the action of the intense heat met with there, they become partially decomposed; oxide of iron and basic sulphides of iron remain. When, at a subsequent period, bricks containing these substances are exposed to the action of the weather, oxidation takes place, sulphate of iron and sometimes of lime are formed, which on crystallizing expand with considerable force and split or crack the brick. From this it is evident that sulphide of iron in any form is not to be tolerated in brick manufacture, and if the earth used in the first place contains much, it must be removed in the preparing process. If permitted to remain, it is impossible to obtain either a durable, or a good coloured brick. CHAPTER VI. MINERALS: THEIR BEHAVIOUR IN THE KILN (_continued_). CALCITE, ARAGONITE, &c. Carbonate of lime may occur in a crystalline form, or as earthy substances, and many varieties of it are found in clays used by the brickmaker. The commonest are calcite, aragonite, and a white earth. Calcite, known also as calc-spar, crystallises in the hexagonal system, though true hexagons are not very common. It occurs principally as rhombohedra and scalenohedra, with variations therefrom; also fibrous, lamellar, granular, compact, nodular, and stalactitic. When pure, calcite is colourless and usually transparent, but when mixed with iron or other mineral colouring matter it commonly assumes yellow and brown tints. Aragonite is also a crystalline form of carbonate of lime, but is by no means as common in Nature as calcite. It crystallises in the rhombic system, which assists the mineralogist to distinguish it from the last-mentioned mineral, from which it differs also in being harder and of higher specific gravity. Aragonite may occur as globular masses, or as incrusting other substances, or in the stalactitic form. It is sometimes white, but more often yellowish, or grey, and it is not, commonly, as transparent as calcite, whilst it often possesses one to two per cent. of carbonate of strontia, or other impurity. It is generally stated that carbonate of lime, when deposited from cold solutions, crystallizes in hexagonal (calcite), and when from warm solutions, in rhombic (aragonite) forms. No doubt, on the whole, that is the case; but we ought not to forget that many marine organisms make their hard parts of aragonite, which, under the circumstances, is certainly not obtained from warm solutions. These crystalline forms of carbonate of lime are both of them found in fossil shells and the like in clays, and in not a few instances the calcareous constituent found in the brick-earth is present almost exclusively in the fossils, which are ground up with the rest in preparing the material for the moulding machine.