Page:Popular Science Monthly Volume 22.djvu/202

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became so completely saturated with it that the innermost fractures resembled the natural asphalt-stone; but it differed from the latter in. this most important property, that it did not crumble to a powder when heated. On the contrary, the absorbed bitumen was expelled by heat, leaving a hard limestone, instead of a calcareous powder. From this it seems probable that the particles of the natural asphalt stone are simply cemented together by the bitumen, and both must have been deposited at the same time.

In most mines the strata dip slightly toward the horizon. They vary greatly in thickness; sometimes there is only one stratum, at others there are several superimposed on each other or separated by strata of harder limestone or shale. At Val de Travers the strata that are worked are from two to six metres thick, and rest on hard, non-bituminous limestone.

In some places it is mined in open trenches, in others by means of shafts and subterranean tunnels. At Val de Travers, Seyssel, and Lobsann, the latter alone are employed. The rock is blasted out with powder, which works better in soft rock than dynamite. The holes are bored with an ordinary hand-auger. At Limmer, owing to the water in the mines, it is necessary to use dynamite.

The percentage of bitumen in the different varieties of asphalt stone is as follows: That from Limmer, 14·3; Val de Travers, 10·15; Lobsann, 12·32; Ragusa, 8·92; Seyssel, 8·15; Vorwohle, 8·50. It is estimated by extracting it from the finely pulverized mineral with carbon disulphide, benzene, or other solvent, and weighing the residue after the solvent has evaporated. The quality of the bitumen is determined by heating it to 430° or 440° Fahr.; the less it loses by evaporation the better its quality. The powdered mineral from which the bitumen has been extracted should be white and soft. If it has a gray color, and feels harsh or sticky, it is of poor quality. Too much dependence can not be placed on chemical analyses, for much depends on the physical properties as well.

The larger portion of the asphalt-stone used in Europe comes from Val de Travers, which produces about 25,000 tons a year; Limmer is not much inferior in its yield, which amounts to about 21,500 tons; Seyssel furnishes 13,000 tons, and Lobsann about 9,000 tons. That which is mined at Limmer and Vorwohle, being very rich, is only used for making mastic.

The first operation to which the asphalt-stone is subjected, when it reaches the factory, is pulverization. For this purpose several different machines are in use, the ordinary stone-breaker being unsuited to the purpose. Four or five of these are figured in Professor Dietrich's new work on "Asphalt Streets,"[1] to which we are indebted for many of the facts in this article.

The powder thus obtained may be employed directly for the com-

  1. "Die Asphalt-Strassen,' E. Dietrich, Berlin, 1882, pp. 207.