Regarding them generally, it may be said that they are remarkable as not containing much acid, practically the whole substance of the rocks (except No. 1) being made of the carbonates of lime and magnesia. In manufacturing bricks of such materials as these, it will be seen that the ordinary methods of brickmaking would not suffice. On heating magnesian limestone, the carbonic acid is driven off, leaving the base behind; it is estimated that the loss of the acid, _plus_ moisture dried out, leads to its reduction in weight of from 40 to 45 per cent., and the shrinkage is from 25 to 35 per cent. If water were mixed with this material, after calcination, strong chemical reactions would result, and of such a nature as to render a coherent mass of the kind required for making bricks impossible. Seeing that water cannot be employed, crude petroleum oil, coal oil, resin oil, &c., have been employed, all of them with more or less satisfactory results. The petroleum, &c., is mixed with the lime, and when the whole is burned the oil passes off, leaving bricks of solid lime. In manufacture it is highly essential to see that the lime is well burned, and it must be fresh, and not have been exposed to a damp atmosphere. An improvement has been effected by mixing from 5 to 7½ per cent. of burned clay, which makes the lime harder after burning. An admixture of from 3 to 5 per cent. of iron oxide also consolidates the lime, though it increases shrinkage. The bricks are commonly made, in the first instance, under hydraulic pressure. The diatomaceous earth known as Kieselguhr, which is used in the manufacture of fire-bricks for chemical works and the like, and which, for the most part, is of German origin, has the following chemical composition:-- _Chemical Composition of Kieselguhr._ Silica 83.8 Lime .8 Magnesia .7 Alumina 1.0 Oxide of Iron 2.1 Organic matter 4.5 Water, &c. 7.1 The reader will perceive that this earth is composed very largely of silica, though there is enough iron, &c., to flux it, at any rate, without material addition. The product is extremely light, and when properly made, Kieselguhr bricks are the lightest known. They are usually of a light yellow tint, with iron spots. The silica is not in a crystalline form, the bulk of the material being composed of the hard parts of microscopic plants known as diatoms; it is more like flint. An earth of a similar character is found in the Isle of Skye, as previously mentioned, though that burns into a redder colour. An infusorial earth from Tuscany is composed of silica 55, magnesia 15, water 14, alumina 12, lime 3, and iron 1 per cent. That also is made into very light bricks. The general principle underlying the method of utilising those earths of organic origin is similar to that of the Dinas bricks, though they do not always require artificial fluxing.