Page:Popular Science Monthly Volume 22.djvu/204

192 pavements, and is much employed in Paris. The powdered rock is used without any addition. It is applied hot, on a prepared bed of concrete, four to seven inches thick, and compressed, with heated rammers and a heated roller, to the thickness of one and a half or two inches. The smooth surface is given by a. heated smoothing-iron. One block of compressed Val de Travers asphalt, two inches thick, laid on a Portland cement concrete foundation seven inches thick, may be seen on Fifth Avenue, between Twenty-sixth and Twenty-seventh Streets.

The third form of pavement, which seems to be one of the best for roadways, is the "Trinidad." It is made of prepared bitumen, i. e., Trinidad asphalt and still-bottoms, mixed with about twice its weight of calcareous marl or powdered limestone. None of the imported asphalt mastics or rock are used in this pavement.

Various imitations of both asphalt and mastic have been palmed off on the public, or substituted by dishonest contractors, some of whom will keep a few blocks of real mastic of a well-known brand lying about, as if they were to be used, while inferior materials are thrown into the caldrons. Some imitations are but little inferior to the genuine, while others are nearly worthless, and have done much to bring asphalt into disrepute. Among the latter are those made in whole or in part of the pitch left in the distillation of coal-tar. Although useful for a great variety of purposes, it will not answer for asphalt pavements. It is usually possible to distinguish good bitumen by its smell when warmed. When heated with excess of concentrated or fuming sulphuric acid for twenty-four hours, and then diluted and filtered, the pure natural bitumen yields a nearly colorless solution, but if pitch is present the solution will be dark-brown or black. Another distinction between real bitumen and coal-tar is found in the solubility of the latter in alcohol, the former being nearly insoluble. If a grain of material that has been heated to 200° C. is pulverized and mixed with 5 c. c. of strong alcohol, the latter will acquire a yellow color and bluish-green fluorescence if there is more than two per cent of pitch present.

There are several uses to which asphalt may be applied, the most important being the one already so often referred to, namely, as paving material. In Paris about thirty-three miles of street are covered with asphalt pavement, more than three fourths of it being the so-called "compressed asphalt," while the remainder is made of cast or mastic asphalt. The use of asphalt pavements for roadways began in Paris in 1854, since which time their use has been steadily increasing until the present time. In London there are about nine miles of asphalted streets. Asphalt pavements have but recently begun to find favor in Berlin, and at the close of 1881 there were only six miles of street paved with it. Of asphalt sidewalks, etc., Paris has three million square metres, equivalent to four hundred miles of walks, seventeen feet wide.