If the clay contains larger proportions of lime or magnesia, a little experience will enable their presence to be detected at once by the way the clay behaves on being placed in contact with water. Pure clay quickly forms a fatty and extremely plastic paste, and sticks closely to the tongue when applied in the dry state. On the other hand, clay containing much lime or magnesia is far less plastic when mixed with water, and the dry clay hardly adheres to the tongue at all. These latter clays are classed as poor or lean, in contrast to the fat, plastic kinds. For certain purposes for which clay is used as pigment, these admixtures are not harmful; whereas others, especially quartz sand and mica, not infrequently present in white clays, constitute a serious drawback. As already mentioned, clay is formed by the weathering of felspar, which is a constituent of granite and gneiss, both rocks composed of quartz, mica and felspar. When the clay has been derived from the weathering of such rocks, it is easy to understand that it may contain admixtures of quartz and mica, which are frequently visible to the naked eye, or at any rate under the microscope. Whereas clay forms a white, amorphous mass, the grains of quartz sand are decidedly crystalline, transparent and of vitreous lustre; the scales of mica, on the other hand, appearing as thin tabular crystals, mostly of a green or brown colour and exhibiting, when viewed at certain angles, a brilliant metallic sheen. Quartz sand can be eliminated from clay without any special difficulty, quartz being of higher specific gravity and therefore settling down quickly, leaving the delicate particles of clay in suspension in the liquid. The scales of mica are harder to get rid of, their tabular form retarding deposition from the suspending liquid; and on this account, several washings are often required to separate them completely. In all cases where clay is to be used as a white distemper, the presence or absence of lime is immaterial; but where it is to be employed for removing grease, lime is a drawback. This is also sometimes the case when the clay is wanted for the purposes of the colour manufacturer. The author has found, by experience, that perfectly pure, white clay forms a good paint, in a vehicle of oil or varnish--a purpose to which it has, so far, been seldom applied, if at all. Such paint is of good covering power, and possesses the valuable property of remaining quite unaffected by atmospheric influences. If, however, the clay contains even but a small quantity of lime, it cannot possibly be used as an oil or varnish paint, for though the freshly made paint has a very good appearance, its character soon changes, turning viscous and suffering a considerable diminution of covering power. Thinning with turps or boiled oil results in the formation of small lumps, so that it is quite impossible to obtain a uniform coating on even a small surface.