uts, ears, and horn pith are also largely used. Cattle, calves,
goats, pigs, horses, and rabbits, all yield characteristic glues.

The best glue is made from hides of oxen, which are soaked in lime
water until fatty or partly decayed matter is eaten out and only the
glue is left. The product is cleaned, boiled down and dried.

The best and clearest bone glues are obtained by leaching the bones
with dilute acid which dissolves out the lime salts and leaves the
gelatinous matters. Such leached bone is sold as a glue stock, under
the name of "osseine." This material together with hides, sinews,
etc., has the gelatin or glue extracted by boiling again and again,
just as soup stock might be boiled several times. Each extraction is
called a "run." Sometimes as many as ten or fifteen runs are taken
from the same kettle of stock, and each may be finished alone or mixed
with other runs from other stock, resulting in a great variety of
commercial glues.

Manufacturers use many tests for glue, such as the viscosity or
running test, the odor, the presence of grease or of foam, rate of
set, the melting-point, keeping properties, jelly strength (tested
between the finger tips), water absorption (some glues absorb only
once their weight, others ten or twelve times), and binding or
adhesive tests. This latter varies so much with different materials
that what may be good glue for one material is poor for another.

Putting all these things together, glues are classified from grade 10
to 160, 10 being the poorest. The higher standards from 60 and upwards
are neutral hide glues, clear, clean, free from odor, foam, and
grease. The lower standards are chiefly bone glues, used for sizing
straw hats, etc. They are rigid as compared with the flexibility
of hide glues. For wood joints the grade should be 70 or over. For
leather, nothing less than 100 should be used, and special cements are
better still.

The best glue is transparent, hard in the cake, free from spots, of
an amber color, and has little or no smell. A good practical test for
glue is to soak it in water till it swells and becomes jelly-like. The
more it swells without dissolving the better the quality. Poor glue
dissolves. Glue is sometimes bleached, becoming brownish white in
color, but it is somewhat weakened thereby.

Fish glue is made from the scales and muscular tissue of fish.
Isinglass is a sort of glue made from the viscera and air bladder of
certain fish, as cod and