SOLVENTS AND DILUENTS 201
definite chemical compounds and the larger part, being compounds of carbon,
are organic. Formerly, of course, most of them had a natural origin, but now far
the greater number comes from synthetic manufacture. Industrially, the petro-
leum hydrocarbons form the most important class and have a definite place in the
practice of painting. These are aliphatic or paraffin hydrocarbons derived from
crude petroleum, a complex mixture containing hydrogen and carbon. Chemically,
they are known as straight-chain hydrocarbons because the carbon atoms of each
molecule appear to be linked in a line by a single bond. This homologous series
of solvents has the general formula, CnH2rH-2- They begin with the simple gas
methane, CEU, and extend to compounds that have thirty or more carbon atoms
which are hard waxes. The lower homologues with five to ten carbon atoms are
most used as solvents. Chemically, they are not active, but have a good solvent
action on fluid oils, fats, bitumen, rubber, and wax, as well as on a few resins
such as dammar. They are flammable and form explosive mixtures with air,
but are not highly toxic.
Coal-tar hydrocarbons, another class of solvents, are those derived from the
destructive distillation of coal-tar, a by-product of coke and coal gas. Aromatic
and benzene hydrocarbons are other names given to them. They contain the ben-
zene ring (see Benzene) and that material is the simplest member of the group.
Toluene and xylene are derivatives. Like the petroleum hydrocarbons, they are
hydrophobia but, chemically, are more active because of the three double bonds
in the benzene ring. Their general solvent action is stronger and they affect most
resins and certain cellulose derivatives, as well as the other materials which the
petroleum hydrocarbons dissolve. The coal-tar group is flammable and generally
toxic.
Chlorinated hydrocarbons make a further class, those obtained by introducing
chlorine into the hydrocarbon molecule. It goes into coal-tar hydrocarbons through
the double bond and into petroleum hydrocarbons by replacement of hydrogen.
Perhaps the most familiar in this class are carbon tetrachloride, chloroform, and
ethylene dichloride. Their solvent power is somewhat stronger than that of the
other hydrocarbons. Like them, they are hydrophobic but, unlike them, are non-
flammable or of very low flammability. They have, however, injurious physio-
logical effects when used in large quantities. Under ordinary conditions, most of
them are stable, but a few decompose and liberate hydrochloric acid in the
presence of light or moisture. They have a high specific gravity.
Turpentine hydrocarbons are known almost entirely through the one product
familiar in painting practice. Turpentine is a mixture of various aromatic hydro-
carbons known as terpenes, and all of them have a ring structure and the em-
pirical formula, CioHie. They differ according to internal grouping of the atoms.
Composition varies with differences in the species of pine from which the original
balsam comes. Turpentine is fairly active chemically because of its tmsaturation,
and has a moderate solvent action, taking effect on fats, soft resins, and liquid