Page:Popular Science Monthly Volume 7.djvu/95

Rh little more than three years in wood pavements, nearly all treated; many of them are now in an advanced state of decay, and, from the degree of preservation after two or three years' use on suburban streets with hardly any wear, one cannot approve of any of the processes applied, since none of them have effectively neutralized the destructive local agencies, or made up for inferiority in the quality of lumber used. Square, polygonal, wedge-shaped, and undressed round blocks, of pine, spruce, and juniper wood, set in rows, interlocking or parted by interstices, upon sand, board, or concrete foundations, were tried, so that all classes of patentees had chances to trot out their hobbies and gratify their passion to serve the community. Though this is an interesting study, we cannot in this place do full justice by entering into details.

The idea of ranking expensive wood pavements, treated and untreated, as valuable standard pavements, where more substantial materials can be procured at the same or lower prices, will, before long, hardly more than elicit a smile from the critical expert.

In this state of the problem, it may be considered as a new epoch in city-life that the increased facilities of commercial intercourse, by cheapening the cost of transportation, have brought a relief within reach, namely, asphaltum for roadways. The nature of asphaltum is frequently misunderstood, because the mineralogist, in speaking of asphaltum, has reference to the brittle bitumen usually found in Nature, while the civil-engineer designates by mineral asphaltum a porous limestone, in combination with tough bitumen. This limestone was primarily impregnated by volcanic action with petroleum, which appears to have oxidized within the structure of the stone by the slow action of many centuries. Thus both ingredients have been united so thoroughly in the asphaltum that neither heat nor water, nor the combined action of both, in causing decay, can render it hard and brittle by abstracting the tough bitumen from the limestone. It is not strange that the efforts artificially to imitate this intimate union have often produced materials with quite different powers of resistance against the various destructive agencies and vicissitudes of climates; and that the lack of durability—not to speak of numerous dead failures in various compounds of raw or treated coal-tar and coal-pitch with coal-ashes, saw-dust, cinders, sand, gravel, etc., commonly called concrete pavements, which have reduced moderately dirty streets, under the influence of the heat of summer, to vast sticky quagmires—has formed a serious drawback to the introduction of better material, and especially the well-tested native asphaltum, which will probably, in our climate, many times outlast the best artificial composition yet known, as it has done in other countries. The admixtures and distillations from coal-tar and pitch have been amply relied upon as the base for artificial concrete, on account of a supposed resemblance to the native asphaltum. This idea, however, is