Page:Popular Science Monthly Volume 7.djvu/96

86 not sustained by modern scientific tests. Seen in thin layers under the microscope, the bitumen, the color of which is otherwise a deep black, shows a transparent yellowish mass, while coal-pitch is visible as a mass of coherent black points on an orange-colored ground. This investigation of the mastic relied upon suffices to explain in the one case the quality of toughness and binding power, and in the other that of brittleness.

Efforts are now being made to produce concrete pavements based on mixtures of silicate of soda with Portland cement. The latter, along with native asphaltum, is undoubtedly the most important modern building-material, but it has its separate province, and lacks just those qualities of the native asphaltum which are so highly appreciated for paving-purposes. It will hardly ever be successful in the long-run when encroaching on the sphere of the competing material which it has fairly outrivaled as a cement for brickwork and masonry, for which, in ancient times, asphaltum enjoyed a just celebrity, as attested by the remnants of the walls of Babylon and Nineveh. This class of pavement has been tested carefully in France with the well-known béton coignet and has failed. While the artificial mixtures soon require expensive surfacing, the native asphaltum when taken up for piping, or otherwise, after many years' wear, may be used again by simply heating and treating it as at first. In this aspect it bears such a relation to the artificial concretes as a copper roof does to a common tin roof.

Nature has unfortunately produced this valuable mineral deposit in but very few cases, and it has not yet been found in America, for the so-called Trinidad asphaltum consists mainly of bituminous scoriae, cemented together with vitrified sand and earth; and even the more esteemed Cuban asphalts contain from 27 to 34 per cent, of earthy substances. The deposits from Tyrimont-Seyssel, on the banks of the Rhone, in France, were the first to be used for pavements. But, as they contain only from 6 to 8 per cent, of bitumen, the powdered rock was found rather too dry, and therefore was superseded by the extensive deposits of the Val de Travers, the most important valley debouching from the Jurassic mountains of Switzerland into the Lake of Neufchâtel. These, with steady march, have conquered the markets of the world. The deposits known as Neufchâtel rock contain, with a constancy not found anywhere else, from 11 to 12 per cent, of bitumen—a most favorable proportion. Besides, they have absolutely greater toughness as a result of their degree of oxidation. They were formerly extensively used as a mastic for sidewalks, and form an excellent material for carriage-ways. They have been used since 1854 in Paris, and since 1868 in the principal thoroughfares of London and other European capitals.

The success of this bituminous rock pavement is by no means due to the lucky hit of one individual; it is the legitimate result of