Page:Popular Science Monthly Volume 80.djvu/340

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contains less than 15 per cent, of asphaltenes will lack cohesiveness and stability or body, while, on the other hand, if it contains less than 70 per cent, of petroleum it will not be sufficiently adhesive. Even with the proper proportion of petrolenes and asphaltenes a bitumen may still be valueless as a cement, if the petrolenes are not of a proper character, that is to say, not sticky. These are all facts to be determined by the chemist, and his contributions to the subject have been of the greatest importance to the development of bituminous highway construction. The characteristics which he determines may be summarized as follows:

1. General Characteristics.—The series of hydrocarbons of which the bitumen is composed for the purpose of comparing it with those in standard materials.

2. Purity.—The amount of bitumen apart from the mineral or other matter, with which it may be contaminated, to regulate the amount of it which should be used under various conditions.

3. Adhesiveness.—Arrived at from a determination of the specific gravity of the bitumen, its solubility in naphtha, the amount of paraffine scale which it contains, this being evidence of the facts that paraffine petroleums are present in the material or absent, and its ductility or extent to which a small test piece can be elongated under tension without fracture.

4. Cohesiveness.—Determined by the percentage of asphaltenes which the material contains, and by the residual coke remaining after ignition of the material in absence of air, which bears a close relation to the percentages of asphaltenes present.

5. Consistency.—Determined by the depth to which a weighted needle will penetrate into the material, under a definite weight, at a definite temperature, during a definite period of time.

6. Viscosity.—Determined by the rate at which the material will flow through an aperture of definite size, at a definite temperature, in a definite period of time.

7. Capacity to Resist Temperature at Which it Becomes Sufficiently Liquid to ie Used in Actual Construction.—Determined by the volatilization of the material when exposed for a definite length of time in a definite amount, to the high temperature at which the materials would be used.

8. Safety.—Determined by the temperature at which the vapor arising from the material at high temperatures, such as those used in manipulating it, will flash or take fire.

Determination by the chemist of the above characteristics and comparison of them with well-known standards enables him to say whether the bitumen in hand possesses those which have been recognized as desirable in similar materials which have been subjected to service tests in actual work with successful results.