ASPHALT. 209 



{technically called "cracking"), and the material is deteriorated. 

 This gives a condition in the product known as "Carbonos," and 

 estimated as being the portion soluble in carbon bi-sulphide but 

 insoluble in carbon tetra-chloride. 



The temperature is maintained at about 700° F. until 

 sufficient of the hydrocarbons constituting the lubricating stock 

 are removed to give a residual pitch of the desired consistency. It 

 was formerly the practice to run down to a low consistency in 

 order to get out the valuable heavy lubricants, and flux back to 

 the desired consistency, imitating the method in vogue with the 

 lake asphalts, but it is now conceded that a better material is 

 obtained if the distillation is stopped when the required consistency 

 is reached. This is determined by the still-man by drawing a 

 sample from the still, cooling it in water, and chewing it. This 

 is a crude method, but it has the advantage that the apparatus is 

 always available, and the temperature of the test always the same 

 A "chewing sample" of known consist encv is supplied by the 

 laboratory for practice, and some still-men get very expert in grad- 

 ing a batch by this method, others pretend to be. When his judg- 

 ment decides by this test that the correct grade has been reached 

 the fire is extinguished, and the still is drawn into the cooling 

 kettle, from which it is barrelled while still warm enough to flow 

 readily. Some still-men draw the fire before the grade is reached, 

 and come to grade by steam alone. A sample is taken during the 

 barrelling process to be tested in the laboratory by the penetro- 

 meter, and the figure obtained is called its "penetration." If 

 it is not of the correct penetration it can be modified, when used, 

 by mixing with another batch either harder or softer as the case 

 may be. 



Asphalt made in this manner is practically 100 per cent, 

 bitumen. The term has no technical value, as the petroleum from 

 which it was made was that also. The operation of distillation is 

 completed in from 24 to 36 hours. On account of this rapidity 

 of manufacture the hydrocarbons are in a state of unstable 

 equilibrium, and during subsequent years are constantly 

 endeavouiing to reach a position of lest. This results in a gradual 

 hardening of the asphalt, and a consequent brittleness that con- 

 duces to disintegration of the pavement. The same action is 

 noticed with coal tar and its derivatives by heating, but in this 

 case there appears to be reason for its success — and its failure. 



Having thus briefly described the sources of asphalt as used 

 in the paving industry, we will pass on to the subject of combining 

 it with the mineral aggregate to construct the modern sheet asphalt 

 pavement. This may be considered under two heads: Ingredients 

 and Combination. 



Ingredients. 



The bitumen in a sheet asphalt pavement amounts to about 

 10 per cent., the remaining 90 per cent, being a well-graded 

 mineral aggregate consisting of sand and a fine dust. This aggre- 

 gate grades from a 10 mesh sieve, having 100 openings to the square 



