When exposed to the atmosphere asphalte gradually assumes a grey tint, by reason of the bitumen evaporating from the surface leaving a thin film of limestone behind. The stone is usually taken from open quarries, but at Val de Travers shafts are sunk and the general treatment is similar to a coal mine. Bitumen, it must be borne in mind, is itself a mineral product found in Trinidad and some other places; it is composed of carbon, hydrogen, and oxygen. The weight of a cubic yard of natural asphalte is about 3874 lbs., its specific gravity is 2·114, but this of course varies with its percentage of bitumen. The following is a test for asphalte given by Mr. Deland in a paper he read before the Institution of Civil Engineers in the year 1880.[91] “A specimen of the rock freed from all extraneous matter, having been pulverised as finely as possible, should be dissolved in sulphurate of carbon, turpentine, ether or benzine, placed in a glass vessel and stirred with a glass rod. A dark solution will result, from which will be precipitated the pulverised limestone. The solution of bitumen should then be poured off. The dissolvent speedily evaporates, leaving the constituent parts of the asphalte, each of which should be weighed so as to determine the exact proportion. The bitumen should be heated in a lead bath and tested with a porcelain or Baumé thermometer to 428° Fahr. There will be little loss by evaporation if the bitumen is good, but if bituminous oil is present the loss will be considerable--gritted mastic should be heated to 450° Fahr. The limestone should next be examined. If the powder is white and soft to the touch it is a good component part of asphalte, but if rough and dirty on being tested with reagents it will be found to contain iron pyrites, silicates, clay, etc. Some asphaltes also are of a spongy or hygrometrical nature. Thus, as an analysis which merely gives so much bitumen and so much limestone may mislead, it is necessary to know the quality of the limestone and of the bitumen. “For a good compressed roadway an asphalte composed of pure limestone and 9 to 10 per cent. of bitumen, non-evaporative at 428° Fahr., is the most suitable. Asphaltes containing much more than 10 per cent. of bitumen get soft in summer and wavy, those containing much less have not sufficient bind for heavy traffic, although asphalte containing 7 per cent. of bitumen properly heated does well for court yards, as it sets hard when cold.” For roadways “compressed” asphalte should be used and not “mastic,” which is only fitted for footpaths, court-yards, etc. Compressed asphalte roadways are constructed as follows: