There is very little to be said concerning the behaviour of free silica--quartz and flint--in the kiln. It is infusible except at higher temperatures than are employed by the brickmaker. But, as we have already remarked, the impurities often present in the minerals form a species of flux which naturally brings them into the range of fusible substances, though even then the temperature required is far beyond what is usually attained in the majority of brickyards, though it might be frequently arrived at in the manufacture of certain fire-bricks. For all ordinary purposes, therefore, quartz and flint may be regarded as infusible. In presence of much lime, iron, or similar substances, however, both of them are readily melted, and it is part of the science of brickmaking to know exactly how much lime, &c., to add to yield the best results. Many brick-earths contain large quantities of the calcareous and ferruginous substances alluded to, and are then capable of being made into bricks direct, without any addition. But although such natural brickmaking earths are frequently employed by the manufacturer, nearly all of them could be made to yield a better brick by a little artificial mixing. We must keep urging this point; there is room for great improvement all round. As with the majority of comparatively refractory substances, the size of the grains and pieces of quartz and flint makes a difference in their readiness to become fusible. The larger the grain the more difficult it is to break down; fusion commences at the outside of a quartz grain, the centre of which may at the same time be comparatively unaffected. By arresting the fusing process, the microscope shows the outside of the grain to have become softened (so much so as to affect its doubly refracting properties), whilst the innermost parts still retain their usual optical characters. MICA. The different varieties of mica are important as rock-forming minerals, but they are not as often met with in brick-earths as is generally supposed, except in insignificant quantity. Some of the purest clays, however, contain a great deal of mica, derived almost directly from the destruction of granite. The two commonest varieties of the mineral are _biotite_ and _muscovite_. BIOTITE MICA.--This mineral, usually known as ferro-magnesian mica, is composed of silicates of magnesia, alumina, iron, and alkalies in variable proportions. It occurs as six-sided plates or irregular scales, usually of a bronze-black colour. Biotite weathers with comparative facility, hence the reason why it is not more commonly met with in brown and other impure clays.