Bischoff's assay is based on the comparison of every clay with one from Garnkirk, in Scotland, which is taken as a type. For this purpose, the clay to be examined is mixed with one, two, three to ten parts of quartz, as the case may be. It is then raised to a known temperature and compared with a piece of the type clay of the same size and shape, which has been submitted to the same If the clay with three parts silica acts like the Scotch clay with one it is called three, and so on. The best and simplest assay seems to be the one made by the blow-pipe, which consists in mixing a small quantity of clay with water, and then spreading it out carefully on a piece of platinum-foil in a very thin sheet, which, when completely dried, is submitted to the flame and compared with clay of known fusibility and prepared in the same way.
For many purposes the density of a clay is an important ele ment of consideration. When strong fire-brick are needed, or glass-house pots, a dense, solid clay 'is desirable. One of the superior qualities of the celebrated Stourbridge clay of Eng land, and that from Coblentz, Germany, as also the Missouri clays, is their comparatively great density. That of the first
named is 2.435 to 2.553 ; that of the Coblentz, 2.229 to 2.266; that of the Cheltenham clay, Missouri, 1.708 to 1.715 ; that of the Evans mine, Missouri, 1.759 to 1.789.
The specific gravity of the clays examined by the State Geologist of New Jersey was determined as follows : A prism about an inch in length was cut off the solid mass. This was covered by a film of paraffine and weighed, first in air, then in water.
A few were taken in this way ; afterward the prisms were placed in water, in a glass vial very little larger than the clay, and then weighed. No water was absorbed by the clay in this modification of the method.
By this method the openness or porous condition, which af fects the density, was taken into account. The ordinary method neglects this condition, and the specific gravity as ob tained is that of the clay, sand, etc., or solid mass, without any reference to the spaces or interstices between the particles of solid matter.