Even under the most favourable conditions, however--when the particles composing the mass require a ¼-inch objective for their elucidation--we find that the best burnt brick is largely made up of them in an unmelted condition. And we should be very sorry to get rid of them; for if they disappeared, the stony attributes of the brick would disappear also, and the general value of the substance would be deteriorated to such an extent that it would be unsaleable as a building material. The brick would nearest resemble a form of slag. All we now insist upon is that in brickmaking a chemical analysis is only useful up to a certain point, beyond which we must appeal to the microscope to aid us, and this in conjunction with as perfect a knowledge as possible as to the behaviour of earths of certain mineral composition when under the influence of high temperatures. In many instances, the value of the brick depends almost entirely on incapacity for fusion on the part of a large proportion of the minerals of which the brick is made. Possibly, a good all-round brick would be where the bulk of its mineral particles were infusible at the temperature employed, and when the remainder were fusible enough to partially run, so as to cement or agglutinate the infusible particles firmly together. In order to bring about such conditions artificially, or to arrive at them even approximately, we must know at least three things, viz.--(1) the nature of the mineral particles involved in the whole operation; (2) their behaviour under high temperatures; and (3) a knowledge of certain branches of metallurgical chemistry. Now, obviously, we cannot undertake to teach even the spirit of what is involved in these three _desiderata_ in a small book like this; but we can, and shall, attempt to do something in that direction, and we must ask the reader’s indulgence to take for granted observations to be occasionally made, in the inevitable prospect of our not being able to explain them at sufficient length. The following are the principal minerals found in clays used in brickmaking, together with their more important attributes from our point of view. KAOLIN.