ood block and stone
block pavements and for expansion joints. These various materials
differ mainly in their consistency at air temperature. (They may
differ widely in chemical composition, but that need not be considered
herein.)

=Water Gas Tar.=--Water gas tar is obtained as a by-product in the
manufacture of illuminating gas from crude petroleum. It is used for
the same kinds of construction as coal tar, and the products utilized
for the several purposes, like the coal tars, differ mainly in
consistency.

=Natural Asphalt.=--Natural asphalt is found in deposits at many
places in the world, existing in beds or pools where it has exuded
from the earth or as veins in cavities in the rocks. It is of varying
composition and consistency, but those kinds in most general use are
solid or very viscous liquids at air temperature. Of the deposits that
have been developed on a commercial scale, the Trinidad lake in the
British West Indies and Bermudez deposit in Venezuela are best known.
Both of these materials are too hard in the natural state to be used
for road construction, and are softened, or fluxed as it is called,
with fluid petroleum oil before being used.

=Petroleum Asphalt.=--Petroleum asphalt is a residue remaining after
the fluid products have been distilled from petroleum. Residues of
this sort are not always suitable for road construction, but a number
of brands of road material are obtained from this source. Oil asphalts
are used for dust layers, for binders for macadam roads, for asphalt
cements for sheet pavement surfaces, and for fillers for block
pavements and expansion joints.

=Mixtures.=--Water gas tars and asphalts are sometimes mixed to
produce road materials, and likewise native asphalts and residues
obtained from petroleum are sometimes mixed to produce asphalt cements
for paving mixtures.

=Classification according to Consistency.=--The various bituminous
materials may be classified according to consistency in discussing the
various uses to which they may be put.

=Road Oils.=--Road oils are fluid petroleum oils of such consistency
that they may be applied cold or by heating slightly. They are used
as dust layers on earth, gravel and macadam surfaces. Their efficacy
depends upon the binding properties of the small amount of asphaltic
material that is contained in the oil.

=Liquid Asphalts.=--These are somewhat less fluid than the road oils,
and must always be heated before application, bu