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Page:Popular Science Monthly Volume 56.djvu/553

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537
MODERN CITY ROADWAYS.

position, and each requires somewhat different treatment. The Bermudez, being richer in bitumen and softer, requires the addition of very little flux. The California deposits furnish their own flux in a liquid asphalt or maltha, which is almost absolutely pure bitumen, and the use of petroleum residuum is thereby avoided altogether.

It has been recognized since 1836 that the bitumen which forms the greater part of natural asphalts can be separated into two substances, which have been commonly known as petrolene and asphaltene,

PSM V56 D0553 Asphalt pavement on clinton ave brooklyn.png
Asphalt Pavement on Clinton Avenue, Brooklyn.

the former of which possesses the cementitious qualities essential to the making of a successful pavement. Instead of the arbitrary names—petrolene and asphaltene—these substances are sometimes more aptly designated as active and inert bitumen. It has been found that of the bitumen extracted from asphalts which have given the most satisfactory results in making street pavements, sixty-nine per cent or more is soluble in petroleum naphtha having a specific gravity of 72° Beaumé.

An asphalt pavement can not be economically kept in good