354 BITUMEN 



to pave the carriage-way near the east end of Oxford Street, and especially at ft moist 

 season, most unpropitious to the laying of bituminous mastic. Being formed of 

 blocks not more than throe or four inches thick, many of which contained much 

 siliceous sand, such a pavement could not possibly resist the crash and vibration of 

 many thousand heavy drays, waggons, and omnibuses daily rolling over it. This 

 failure can afford, however, no argument against rightly-constructed foot-pavements 

 and terraces of asphalt. Numerous experiments and observations have led mo to 

 conclude that fossil bitumen possesses far more valuable properties for making a 

 durable mastic than the solid pitch obtained by boiling wood or coal tar. The latter, 

 when inspissated to a proper degree of hardness, becomes brittle, and may be readily 

 crushed into powder ; while the former, in like circumstances, retains sufficient tena- 

 city to resist abrasion. Factitious tar and pitch being generated by the force of fire, 

 seem to have a propensity to decompose by the joint agency of water and air, whereas 

 mineral pitch has been known to remain for ages without alteration. 



Bitumen alone is not so well adapted for making a substantial mastic as the native 

 compound of bitumen and calcareous earth, which has been properly called asphaltic 

 rock, of which the richest and most extensive mine is unquestionably that of the 

 Val de Travcrs, in the canton of Neufchatel. This interesting mineral deposit occurs 

 in the Jurassic limestone formation, the equivalent of the English oolite. The mine 

 is very accessible, and may be readily excavated by blasting with gunpowder. The 

 stone is massive, of irregular fracture, of a liver-brown colour, and is interspersed with 

 a few minute spangles of calcareous spar. Though it may be scratched with the nail, 

 it is difficult to break by the hammer. When exposed to a very moderate heat, it ex- 

 hales a fragrant ambrosial smell, a property which at once distinguishes it from all 

 compounds of factitious bitumen. Its specific gravity is 2' 11 4, water being I'OOO, 

 being nearly the density of bricks. It may be most conveniently analysed by digesting 

 it in successive portions of hot oil of turpentine, whereby it affords 80 parts of a white 

 pulverulent carbonate of lime, and 20 parts of bitumen in 100. The asphalt rock of 

 Val de Travers seems therefore to be far richer than that of Pyrimont, which, ac- 

 cording to the statement in the specification of Claridge's patent of November, 1837, 

 contains ' carbonate of lime and bitumen in about the proportion of 90 parts of car- 

 bonate of lime to about 10 parts of bitumen.' 



The calcareous matter is so intimately combined and penetrated with the bitumen 

 as to resist the action not only of air and water for any length of time, but even of 

 muriatic acid; a circumstance partly due to the total absence of moisture in the 

 mineral, but chiefly to the vast incumbent pressure under which the two materials 

 have been incorporated in the bowels of the earth. It would indeed be a difficult 

 matter to combine, by artificial methods, calcareous earth thus intimately with bitu- 

 men, and for this reason the mastics made in this way are found to be much more 

 perishable. Many of the factitious asphalt cements contain a considerable quantity 

 of siliceous sand, from which they deriva the property of cracking and crumbling down 

 when trodden upon. In fact, there seems to be so little attraction between siliceous 

 matter and bitumen, that their parts separate from each other by a very small disrup- 

 tive force. 



Since the asphalt-rock of Val de Travers is naturally rich enough in concrete bitu- 

 men, it may be converted into a plastic workable mastic of excellent quality for foot- 

 pavements and hydraulic works at very little expense, merely by the addition of a very 

 small quantity of mineral or coal tar, amounting to not more than 6 or 8 per cent. 

 The union between these materials may be effected in an iron cauldron, by the appli- 

 cation of a very moderate heat, as the asphalt-bitumen readily coalesces with the tar 

 into a tenacious solid. 



The mode adopted for making the asphalt pavement at the Place de la Con- 

 corde in Paris was as follows : The ground was made uniformly smooth, either in 

 a horizontal plane or with a gentle slope to carry off the water ; the curb-stones 

 were then laid round the margin by the mason, more than 4 inches above the level of 

 the ground. This hollow space was filled to a depth of 3 inches with concrete, 

 containing about a sixth part of hydraulic lime, well pressed upon its bed. The sur- 

 face was next smoothed with a thin coat of mortar. When the whole mass had be- 

 come perfectly dry, the mosaic pattern was set out on the surface, the moulds being 

 formed of flat iron bars, rings, &c., about half an inch thick, into which the fluid 

 mastic was poured by ladles from a cauldron, and spread evenly over. 



The mastic was made in the following way : The asphalt rock was first of all 

 roasted in an oven, about 10 feet long and 3 broad, in order to render it friable. 

 The bottom of the oven was sheet-iron, heated below by a brisk fire. A volatile 

 matter exhaled, probably of the nature of naphtha, to the amount of one-fortieth the 

 weight of asphalt ; after roasting, the asphalt became so friable as to be easily reduced 



