1 88 PAINTING MATERIALS

chloride (sal ammoniac or sal ammoniacum), a fixed salt of ammonium, was, how-
ever, known to the ancients. The gas (NH3) was first isolated by Joseph Priestly,
English chemist and discoverer of oxygen, in 1774. Until recently, industrial
ammonia was a by-product of coal through dry distillation in gas works and coke
ovens. Now it is made synthetically from hydrogen and nitrogen.

Amyl Acetate (CHsCOOCsHn). At one time this ester of acetic acid and amyl
alcohol was prepared in large quantities from fusel oil, a residue obtained from the
rectification of alcohol. A more uniform product is made synthetically. It is a
clear liquid, with a penetrating odor (giving it the common name, banana oil),
which is overpowering in high concentrations. The boiling point is I48°C. and the
flash point is high, yj°C. In 1882 it was introduced as a solvent for cellulose nitrate
and for many years continued to be the leading solvent for that material. Now it
is largely replaced by butyl acetate. The amyl acetate is a good solvent for many
resins and is miscible with linseed oil. It mixes with water only very slightly.
Much of the synthetic product, composed of five isomeric amyl acetates, is sold
as Pentacetate. There is no record of experimental data on the action of this long-
standing solvent on resin and linoxyn films.

Amines (see Solvents, classification).

Banana Oil (see Amyl Acetate).

Benzene (benzol) is the simplest of the aromatic series of organic compounds.
This is a hydrocarbon with the formula, C6H6, and is usually represented as having
the ring or cyclic structure shown below. It is a clear, colorless liquid, with a

H
C

HC CH

v
H
peculiar odor. It boils at 80° to 8i°C., is highly flammable, and, being an un-
saturated compound, it burns with a sooty flame. It is miscible with most organic
solvents but practically not with water.
The benzene of commerce is largely obtained by the destructive distillation of
coal tar, and the bulk of it comes over in the first or ' light oil * fraction. This is
further fractionated to give the industrial product commonly known as benzol, a
mixture of about two thirds benzene with one third toluene and other benzene
homologues. Among impurities, it contains thiophene, a sulphur compound, and
pyridine. Further fractional distillation of benzof and special chemical treatment
to remove impurities, combined with filtration and crystallization below 5*5°C.
(its solidifying point) yields the pure benzene.
The toxicity of its vapor prevents benzene from being widely used in coating
and cleaning compounds, though it is an effective solvent for a wide variety of