Summing up the general characteristics of these slates from the chemical aspect, one would say that none of them are very suitable for high-class bricks. No. 2 is the best. Several minor differences will be observed between the results quoted and those referring to ordinary brick-earths--in particular, the distribution of the alkalis. A general impression is abroad that any purple slate will do for brickmaking, and manufacturers do not yet seem to have realised that the chemical nature of slates is as variable as of brick-earths. That may account for the difficulties experienced in many cases in turning out a satisfactory material. The microscope is of much use in this connexion, however, and the practical effects of chemical analyses are not always as bad as they seem at first sight. The remainder of this chapter will be devoted to the consideration of rarer kinds of brick-earth and other raw earths used principally in the manufacture of bricks for special purposes, or as pointing to certain anomalies. As an example of what some manufacturers can do, we may quote the chemical composition of a peculiar brick-earth employed in Zurich, in Switzerland:-- _Chemical Composition of Brick-earth, Zurich._ ----------------------------------------+------ |Yellow| Blue |Clay. | Clay. +------+------ Carbonate of Lime |23.68 |27.80 „ „ Magnesia | -- | 5.7 Other carbon dioxide | 2.85 | 1.55 Silica |42.39 |38.25 Alumina |18.16 |12.44 Iron oxide | 3.66 | .73 Lime (as silicate) | -- | 1.85 Magnesia | -- | .15 Potash | 2.14 | 1.54 Soda | 1.27 | 3.05 Moisture (at 100° C.) | 1.27 | 1.37 Water, &c., chemically combined | 3.85 | 4.72 ---------------------------------+------+------