MODERN CITY ROADWAYS. 535 



as before it was disturbed, though usually the product of several 

 mines is mLxed in order to obtain the best percentage of bitumen, 

 but nothing is added to or taken from the bituminous rock. In 

 the pavement usually laid in America, on the other hand, only a 

 small proportion of the material is brought from the asphalt depos- 

 its, the principal part of it (sand) being obtained near at hand. 

 In the one case the cost of long ocean or rail transportation has 

 to be paid on the entire mass forming the pavement, while in the 

 other this expense attaches to but from twelve to fifteen per cent 

 of the material. This, of course, is a great advantage, and at 

 recent prices it is scarcely possible for the European rock asphalts 

 to compete with the artificial ones. 



The making of a pavement from one of the standard asphalts 

 may be briefly described as follows: The material as found in Xa- 

 ture has this composition: 



Bitumen 38 . 14 per cent. 



Organic matter, not bitumen 7.63 " 



Mineral matter 26.88 " 



Water 27 . 85 " 



100.00 



This is cooked until the water has been driven off, and some of 

 the mineral matter has settled. 



The above analysis is of Trinidad Pitch Lake asphalt, and is a 

 particularly favorable result. This material is too hard for use in 

 making a pavement, and it has to be softened or fluxed by the addi- 

 tion of something which will accomplish this purpose. In order 

 to do this there is usually added to each one hundred pounds of 

 refined asphalt about eighteen pounds of heavy petroleum oil. 

 After this addition w^e have the asphaltic cement ready to combine 

 with mineral matter, which is so selected that when asphaltic ce- 

 ment is added at the rate of about seventeen pounds of the cement 

 to eighty-three pounds of the other all the particles Avill be coated, 

 and more could not be added without making the pavement too soft. 

 What is found to accomplish this best is fine stone dust and sand. 



The asphaltic cement and sand are heated separately to about 

 300"^ F. The stone dust is then added to and mixed with the hot 

 sand in the proportion of from five to eighty in the case of fine, well- 

 graduated sand, to fifteen to sixty-seven for coarse sands, having 

 less variation in size. The asphaltic cement is then added, and the 

 materials are mixed to a homogeneous mass, which is ready to be 

 taken to the street. It should reach there at a temperature not 

 less than 250°, and is spread with hot iron rakes so as to give usually 

 a thickness of two inches after consolidation. After spreading, it 

 is rolled with a hand rollel-, after which a small amount of hvdraulic 



