Page:Popular Science Monthly Volume 22.djvu/206

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made in strips twelve, sixteen, twenty, or twenty-four inches wide, to correspond with walls made of three, four, five, or six courses of brick. Its superiority to ordinary bitumen depends on the fact that it will not crack, like the latter, from unequal settling of the walls. Damp resisting solutions are also sold for coating damp walls.

Asphalt mastic is much superior to tar for roofing purposes, owing to its fire-proof qualities, and its use for this purpose is rapidly increasing. At the present writing it is being applied to the Welles Building, at the lower end of Broadway. It is said that, when a building covered with such a roof burns, the falling roof acts like a blanket in smothering and extinguishing the flames.

Asphalt possesses another valuable property, that of absorbing vibrations, and is hence useful for foundations of machinery running at high speeds. A block of bituminous concrete weighing forty-five tons formed the foundation of the Carr's disintegrator which made fourteen hundred revolutions per minute at the Paris Exhibition. It would seem to be especially adapted to serve as foundations for the high-speed steam-engines used for generating electricity.

Asphalt forms an excellent insulator for electricity, but, as other and cheaper materials may be employed, its use will not be so extensive in this field.

The origin of asphalts is unknown, but several theories have been advanced in regard to it. Professor J. S. Newberry believes that they are the more or less perfectly solidified residual products of the spontaneous evaporation of petroleum. If we accept this theory (and many do not), we are but one step nearer a solution of the problem, for the origin of petroleum itself is still unknown. Some think that the bitumen was formed first, and the limestone deposited in it; others, that the liquid bitumen was forced into the pores of limestone already in existence; while a third hypothesis assumes that they were formed simultaneously, the bitumen from the organic matter, and the lime from the shells of some ancient mollusks. The last-named theory seems to have some support in the abundance of fossil ammonites met with in the mines at Limmer; the experimental attempts to impregnate the rock artificially, as above described, render the second hypothesis improbable, although its occurrence on the Dead Sea and in Trinidad is in its favor. No explosive gases are met with in the mines of Val de Travers, Seyssel, and Lobsann, so that open lights are used; but at Pechelbronn, a few miles from Lobsann, several explosions have occurred. Although these were attributed to marsh-gas, they were more probably due to the vapors of the lighter constituents of petroleum, with which the bituminous sands of that locality seem to be saturated.

There are several circumstances which indicate that bitumen and asphalt are more nearly related to petroleum than to coal-tar, and that, whether asphalt was made from petroleum or not, they have a similar