| Carbon, per cent. | Hydrogen, per cent. | Oxygen, per cent. | Nitrogen per cent. | |
| Viscous bitumen of Bechelbronn | 88·0 | 12·0 | — | — |
| Virgin bitumen of Bechelbronn | 88·0 | 11·0 | — | 1·0 |
| Liquid bitumen from Hatten, Lower Rhine | 88·0 | 11·6 | — | 0·4 |
| Solid bitumen of Coxatambo, near Cuença, in Peru | 88·7 | 9·7 | 1·6 | |
Annexed is a table of the analysis of several asphalts, centesimally represented:—
| Bitumen of Bastennes. | Bitumen of Pont de Chateau, Auvergne. | Bitumen of Abruzzi. | Bitumen of Monastier, Haute Loire. | |||
| Crude. | Pure. | Crude. | Pure. | |||
| Oily matters, Bitumen | 20·0 | — | — | — | — | 7·0 |
| Carbon, Bitumen | 3·7 | 76·13 | 77·5 | 77·64 | 81·8 | 3·5 |
| Hydrogen | — | 9·41 | 9·6 | 7·86 | 8·4 | — |
| Nitrogen | — | 12·66 | 12·4 | 1·02 | 1·0 | — |
| Oxygen | — | 0·5 | 8·35 | 8·8 | — | |
| Water | — | — | — | — | — | 4·5 |
| Gas and vapour | — | — | — | — | — | 4·0 |
| Quartz sand and mica | 76·3 | — | — | — | — | 60·0 |
| Clay | — | — | — | — | Ferrug. 21·0 | |
| Ashes | — | 1·80 | — | 5·13 | — | — |
| ——— | ——— | ——— | ——— | ——— | ——— | |
| 100·0 | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 | |
The solid bitumens are now extensively employed in the manufacture of bituminous mastic or cement and similar compositions, which are used for the lining of water-cisterns, and for various other hydraulic purposes; as also for roofs, floors, roads, pavements, &c. For the last purpose the native varieties of ‘asphaltic rock,’ consisting of a mixture of bitumen and calcareous earth, when tempered with a proper quantity of crushed granite, or calcareous sand or gravel, is found to be the most substantial and durable. The plan followed in laying down such pavements in Paris, where they have been the most extensively adopted, is—The ground having been made uniformly smooth, is edged, in the usual manner, with curb-stones rising about 4 inches above its level, and then covered, to the depth of 3 inches, with concrete (made with about 1-6th part of good hydraulic lime), which is well pressed upon its bed, the surface being subsequently smoothed over with a very thin coating of hydraulic mortar. On this, when perfectly dry,[187] the ‘bituminous mastic,’ rendered semi-fluid by being cautiously heated in a suitable iron cauldron,[188] is evenly spread over so as to form a layer three quarters of an inch, or for less solid work, half an inch thick. Some coarse sand is lastly sifted over and pressed down on the surface, when the work is complete; and in a few days the pavement becomes sufficiently compact and solid to be thrown open to foot passengers.
[187] On this point depends the success of the work. Absolute dryness is a sine quâ non in the process. The mastic must also be laid down in dry weather. If laid in wet, damp, or even foggy weather, it will be liable to separate from its bed, and gradually to break up. This is why so much of the asphalt and bituminous pavement laid down in London has proved a failure.
[188] It is here that the mixture of the bitumen (previously crushed sufficiently small to pass through meshes 10 to the inch) is made with the sand or gravel; a small portion of mineral tar or coal-tar (3 to 7 or 8%) being commonly added to promote their fusion and complete union.
An important precaution to be observed in
making asphalt pavements or roads is to boil the bitumen which is employed thoroughly, so as to expel the water and volatile oils, which if allowed to remain are found to render the mastic more liable to be affected by the extremes of heat and cold, as well as less able to stand the wear and tear of traffic.
Claridge’s Process. This consists in fusing the blocks of mastic in a suitable boiler, similar to that seen in fig. 1, and in adding a quantity of mineral tar, in the proportion of 1 lb. to every cwt. of the mastic. The tar is first fused in the boiler, 56 lbs. of the mastic are then introduced, and the whole repeatedly stirred so as to prevent the formation of a deposit. When the contents of the boiler are melted, the cauldron is covered over for a quarter of an hour, after which the remainder of the mastic is added, and its fusion proceeded with as before, the process being repeated until the boiler is full, allowing an interval of from ten to fifteen minutes between each operation.
When the mastic is sufficiently fluid it will drop freely from the stirrer, and jets of light smoke are observed to issue from it. If stiff mastic be required, the proportion of tar is lessened, and a quantity of coarse grit or river sand, to the amount of 20 or 30 lbs. to the cwt., is added.