Asphalt of the best quality may be known by the following conditions :—The grain is fine and homogeneous, and does not exhibit a particle of ordinary or white limestone. The rock is often lined with streaks of a darker colour than the rest, which give it very much the appearance of a tiger's skin. Other samples contain crystals of carbonate of lime, impregnated with bitumen like the rest; these crystals are sometimes of a considerable size. All these varieties are perfectly good so long as they are completely penetrated by the bitumen. Bad qualities may be recognized as follows ;—Sometimes the rock is regularly impregnated, but the proportion of bitumen is as low as 6 per cent., when it can be worked only with much difficulty. Sometimes the limestone is very hard and much cracked, the clacks being filled with bitumen, so that, when broken, the fracture appears brownish black, like the good samples, but when examined with the microscope, the impregnation is seen to be very incomplete. Samples of this kind are frequently met with in Auvergne. Sometimes the limestone, while it appears rich in bitumen, contains clay, which, being impenetrable, spoils the homogeneity, and causes the fissures so often seen in streets paved with the material; the presence of clay in the sample is easily recognized. Some bituminous limestones, that of Lobsann, for instance, contain an oily principle beside the bitumen, which renders them greasy and spoils the consistence of the mastic made from it ; this oil may be removed by distillation, after which the asphalt is fit for use. When asphaltic rock has been long exposed to the air, the bitumen on the surface dries up, to a depth of about of an inch. This desic cation, which is due to the slow evaporation of the bitumen, is sufficient to discolour the asphalt so much that it becomes similar in appearance to ordinary limestone: blocks of this kind, which are suspected to contain bitumen, must be broken up to ascertain the colour of the interior. This evaporation rarely extends farther into the rock than of an inch. By means of these indica tions, it is easy to distinguish between good and bad asphalt, and they will be found very useful to those who have occasion to seek for this valuable mineral. To such also the following hints may be of use : (I) To search only in calcareous districts ; (2) not to be content with the exterior colour of samples, hut to break into them and inspect the interior ; (3) to examine the character of the mineral, and to determine tho proportiou of bitumen it contains by a process to be described hereafter.
The discovery and application of asphalt to industrial purposes dates from the remotest ages. Many ancient Egyptian bnildings bear traces of its use as a cement. D'Eyrinis, the discoverer of the Val-de-Travers mine, and author of a work on the subject published in 1721, affirmed that Noah's ark was coated with this material, and also that the Tower of Babel was probably cemented with it, basing these assertions upon the proximity of the mine of Siddim. These statements are not, however, worthy of credence, since asphalt cannot properly be used as a cement, especially in a warm climate, except for the foundations of huildinge. At any rate it ie certain that it was much used by the anoients for a variety of purposes, examples of its use being frequently found iu Eastern couutries. After this time, it was lost sight of, and does not appear to have been employed at all throughout the Roman era. In fact, it did not reappear until the eighteenth century, when D'Eyrinis, a professor of Greek, while making a gological excursion in the Val-de-Travers, in the oanton of Neuchatel, discovered in 1710 a deposit of asphalt, which seemed to him to be identical in character with that found in the East. On making experiments, he found that the deposit consisted of limestone impregnated with bitumen, and that, by boiling, it afforded a mastic which closely resembled the Babylonian cement. The enthusiasm of D'Eyrinis pa-sed all bounds, and from the pamphlet which he wrote some years afterwards, it would be supposed th et there could he no possible purpose for which the mineral was unfitted. The pamphlet, however, hinted obscurely at some of the applications which have since been made, though certainly not that which has since proved tho greatest success, viz. its use for paving purposes. The Val-de-Tracers mine was, for nearly a
century, the only known source of asphalt. The first asphalt pavements were constructed in Paris in 1838, and about this time another mine, that of Seyssel, on the Rhone, entered into rivalry with it ; both were worked for a long time, and enjoyed a period of very great prosperity. But as often happens to new industrial schemes carried on on so gigantic a scale, it fell into tho hands of speculators, whose main object was not the successful working of the mine, but immediate pecuniary gain. "A ring" was formed which, in a few months, raised the price of the shares from 500 francs to 13,000 francs. It is scarcely necessary to say that this did not last, and in a short time the 13,000 franc shares ware being offered at 25 francs each I Asphalt, however, was destined to overcome these difficulties, and although it remained for some time iu the hands of speculators, it eventually took its proper place as an important and profitable industry. At the present time, many of the streets of the principal towns of Europe are paved with it. Paris notably has been paved almost exclusively with it, and it has been laid down in many of the finest streets and squares in London.
The chemical composition of asphalt is variable, as regards, at. least, the relative proportions of limestone and bitumen. The purest varieties, ouch as those of Seyssel and Val-le-Travers, contain nothing but these two substances, in about the proportions already indicated. Those which are less pure, as, for example, the bituminous limestones of Auvergne, have been impregnated by volcanic agency, and contain, besides these, clay, silica, magnesia, salts of iron, &c. The Auvergne samples contain also traces of arsenic. As a general rule, it may be stated that samples of asphalt are valuable in proportion as they are free from these foreign matters. It is seldom necessary to make a qualitative analysis of asphalt, the constituents of each separate variety being perfectly well known. But it is often required to determine the proportion of bitumen entering iuto their compo sition. This determination, moreover, has constantly to be made by persons unfamiliar with the processes of chemical analysis, and it is therefore deemed desirable to describe here a process recommended by Malo, which applies equally to all bituminous substances. A quantity (about 200 grm.) of the substanoe is reduced to a fine powder, and dried by exposing it in a current of air heated to a temperature higher than 110°, but not above 150', since above this temperature the bitumen may be altered by the vaporization of certain essential oils. After well mixing this powder, about 100 grm. is taken and placed in a beaker. About 100 grin. of pure carbon disulphide is then poured upon it, and the mixture is well stirred with a glass rod. After resting a moment, it is poured into a weighed filter, having another beaker placed beneath. More carbon disulphide is poured upon the limestone remaining in the first beaker, well stirred, allowed to stand, and the clear portion again added to the filter ; this is continued until the powdered limestone is perfectly white, and the last portions of carbon disulphide added exhibit no tinge of brown. The limestone is then dried whilst the liquid iu the filter is running through. When perfectly dry, the limestone and the filter are weighed together, and after deducting the weight of the filter, the weight of the washed limestone is obtained, and, by difference, the weight of the bitumen removed by the carbon disulphide. The following is an example of such a test :— From which it will be seen, by a simple calculation, that the proportion of bitumen contained in the limestone was per cent. The correctness of the result may be confirmed by evapo rating iu a water-bath, at 70°, the carbon disulphide. This is volatilized at 48° ; the bitumen remains as a residue, and may be collected and weighed, the percentage being thus determined directly.