The practice resorted to in various parts of the world of making bricks from slate _débris_, although not hitherto adopted to any large extent in this country, merits some description in this place. Slates may be regarded as a highly compressed clay, the original structure of which has been materially modified by the great pressure exerted during their manufacture in Nature’s laboratory. To all intents and purposes they are silicates of alumina, _plus_ iron, lime, magnesia, and so on, and have, practically, the same range of variation as have ordinary clays. But during their manufacture, and subsequently, certain adventitious mineral matter has been frequently introduced, as may be gathered from the following results:-- _Chemical Composition of Slates._ -------------------------+-------+-------+-------+------- | 1 | 2 | 3 | 4 +-------+-------+-------+------- Silica | 60.50 | 60.15 | 48.00 | 50.88 Alumina | 19.70 | 24.20 | 26.00 | 14.12 Iron (protoxide) | 7.83 | 5.83 | --- | 9.96 „ (sesquioxide) | --- | 1.82 | --- | --- „ | --- | --- | 14.00 | --- Lime | 1.12 | --- | 4.00 | 8.72 Magnesia | 2.20 | --- | 8.00 | 8.67 Potash | 3.18 | --- | --- | .88 Soda | 2.20 | --- | --- | --- Alkalis (not determined) | --- | 4.28 | --- | --- Carbon dioxide | --- | --- | --- | 6.47 Water, &c. | 3.30 | 3.72 | --- | --- -------------------------+-------+-------+-------+------- _Analysis No. 1_ refers to a blue Welsh roofing slate of Cambrian age. It is quite certain that the large proportion of alkalis present would render this material unsuitable for brickmaking, except for the commonest kinds of bricks. The iron, again, is very large in quantity, whilst the amount of alumina is low. We could not recommend this slate for good bricks under any consideration. _Analysis No. 2_ is of a dark-blue slate from Llangynog, in North Wales. The amount of iron present is high, but from the low content of alkalis this material, under proper treatment, should make fairly good bricks. The ferruginous constituent is too powerful, however, for fire-bricks to be made of this slate. _Analysis No. 3_, of a purple slate from Nantlle, shows a remarkable diminution in silica and a corresponding increase in iron. Lime and magnesia being present to such an enormous extent, taken in conjunction with the iron, would render this slate absolutely useless for brickmaking. There is not a redeeming feature about it. _Analysis No. 4_, which refers to a green Westmorland slate, has a low percentage of alumina and very large quantities of iron, lime, and magnesia. Only bricks of an exceedingly inferior quality could result from such material.