CLAY, as commonly understood, means earth of sufficient ductility to allow of its being kneaded by the hand into useful shapes or forms, and ranks as a _raw_ material, or one not worked up or prepared for use. Some clays are soft, others are indurated, or hard and rocky: but all have, nevertheless, been in one sense prepared by poundings, washings, and mixings, carried on by Nature on a much larger scale than that on which they are now fitted for use. They differ in quality, in degree of firmness, and in colour, and show certain relationships by which it is clear that they are derived from sand, just as sand is derived from a hardy race of pebbles, which in turn bear a close relationship to rocks, from which undoubtedly they are also derived. Surface clays, used for making inferior bricks and tiles, whose earthy odour gives evidence of alumina, are often derived from red sandstone rocks, which have been ground down by machinery of waves or streams; whilst the deeper coal-measure clunches, used for firebricks and pottery, were originally the sediment thrown by rivers at their embouchures into inland lakes or seas. Common red clays, deriving their colour from iron, have many impurities, and contain a large percentage of alumina. Fire-clays are nearly free from iron, and contain a large amount of silica, whilst china-clay, or _kaolin_, contains felspar, sometimes with the impurities of soda and potash. Let us first take brick and tile Clays. Of the three substances expressed by the three words of four letters—clay, coal, and gold, we question whether the first does not rank highest in importance. The latter may be the most coveted, but the former, we imagine, contributes most to the conveniences and comforts of mankind. It is in one form or another universally attainable. There is a great difference in its qualities; and when it is remembered that a porous brick made from bad clay will hold nearly a quart of water, the advantage of good clays producing good bricks which will protect health and property from the injurious effects of a fickle climate, becomes apparent. In addition to ordinary clays used for the manufacture of building materials, we have throughout the whole coalfields of Shropshire and Staffordshire superior fire-clays, which occur in a tough indurated state, and are known by the familiar name of clunches. They possess but a small portion of iron, which gives a red colour to ordinary clays, at the same time that they are distinguished by an almost entire absence of lime and alkalies; yet contain, on the other hand, that proportion of silica and alumina which although they cannot be melted by the strongest heat, form ingredients which during the process of burning combine to form an artificial stone capable of sustaining great heat. The following is an analysis of one of these fire-clays:—