The reader may be tempted to enquire, What is the use of knowing the micro-structure of a brick? We have anticipated the question to some extent in dealing with the structure of brick-earths, but it may be well to enlarge upon it here. In the first place, the study of the minute structure enables the manufacturer to ascertain whether the brick is thoroughly and homogeneously burnt. It tells him whether the materials mixed together in the earlier stages of manufacture were thoroughly incorporated or not, whereby, if need be, he can improve that part of the process. In carefully examining what the average manufacturer would call a well-burnt brick, the microscope assists us in perceiving that it is often anything but well burnt, small local patches--“tears”--of semi-vitrified matter being observed, which should not exist, of course, in a perfectly homogeneous brick. And if the brick is not homogeneous, it suffers in respect of its strength as a whole, and in the majority of cases its colour is not uniform. To arrive at the cause of this lack of uniformity is to indicate the manner in which the manufacture of the brick may be improved, and the microscope often enables us to arrive at a satisfactory solution of the problem. From a chemical standpoint we know that a high percentage of iron in the average brick-earth is not conducive to the production of a good brick. In the same manner by “rule of thumb” we learn that a high percentage of lime prevents the manufacture of the raw material into a fire-brick, unless, indeed, we are making basic bricks. The chemist tells us also of the respective values of potash and soda. Too much iron will cause the brick to “run”; salt has a similar effect; but beyond this the chemist cannot go, except that in the broad sense he explains what unions take place to produce such results. The microscope, on the other hand, enables one to see exactly what has taken place; the deleterious constituents are detected at their work, and careful chemical investigation teaches us what to add to the brick-earth to neutralise the effects observed; for it is only from its effects that the artificial constitution of the brick-earth can be properly regulated.