Two distinct forms of precipitated calcium sulphate are met with in commerce, differing from each other by their microscopical features, the one consisting of flat tabular crystals (Fig. 41), the other of fine needles (Fig. 42). Another form, erroneously called precipitated pearl-hardening, is also sold: it consists of the finely ground native mineral.
FIG. 40.
FIG. 41.
Some of the finer qualities of paper are made without addition of any loading material whatever, though such papers are of course the exception. The proportion of clay or {132} other material that can be put into a fibre depends to a certain extent upon the nature of the fibre, and upon the degree of fineness to which it is reduced in the beater. The amount added by different makers varies considerably, from {133} two or three per cent. to twenty, and even in rare cases to thirty per cent.
FIG. 42.
FIG. 43.