In some instances, partial fusion is so well exemplified (especially in bricks from fairly pure china clay), and the brick, after being burnt, has been permitted to cool so slowly, that devitrification has set in, when we are presented with aggregates of crystallites closely resembling the “felspathic matter” of petrologists. That is a circumstance which the maker should note well, for he has burnt the brick to the best advantage, and it is not then so brittle as it might have been had more “glass” made its appearance in the section. Prolonged heat, just above the agglutinating point, has accomplished this, and the microscope here clearly shows the advantage of allowing the kiln to cool slowly, and to permit the lapse of several days in the operation. CHAPTER XII. THE MICRO-STRUCTURE OF BRICKS (_Continued_). Turning now to the actual appearance of minerals commonly found in bricks as they are examined under the microscope, we may remind the reader, that the physical aspect of the majority of them has already been described in those chapters dealing with the “Mineral Constitution of Brick Earths” and “Minerals: their behaviour in the Kiln,” and the particulars that follow may be read in conjunction with what was there said. It will be convenient now to describe the appearance of certain well-known minerals, as they are seen (A) in reflected light and (B) in thin sections in transmitted light, whilst the latter will be subdivided into _1_ denoting the phenomena observed in ordinary light, and _2_ in polarised light. To save repetition, the letters and figures will be used to denote the methods of examination as indicated. QUARTZ.--Present in nearly all rubber-bricks, and in the vast majority of common stocks, as well as in vitrified goods and fire-bricks. In the last mentioned, the grains are usually partially agglutinated, and are extremely minute. A. As more or less rounded, or sub-angular fragments, white and crystalline, like clear window glass. B. 1--Clear white, often broken up by thin hair-like lines running in various directions, and rows and patches of minute specks, which, as previously remarked, have been shown to contain fluid, &c. 2--On revolving the stage of the microscope, the crystals are usually seen to present beautiful, clear transparent colours, which in characteristic sections are very vivid--red, blue, yellow, &c. FLINT.--Found in the same class of bricks as quartz. A. Bluish horn colour; irregular fragments and splinters. B. 1--Translucent; often melted more thoroughly than quartz in hard burnt bricks; colourless. 2--Opaque unless in some such form as chalcedony, when an extremely minute granular aspect results, becoming slightly transparent. Melted portions always opaque. FELSPAR.--The alteration which the different kinds of felspar have undergone in a hard burnt brick, when present, render it almost impossible to recognise them specifically.