SCIENCE IN HIGHWAY CONSTRUCTION 335 



more particularly the sulphur compounds, of which it is composed. 

 The consistency of such material and its suitability in this regard for 

 use as a road binder is further dependent on the relative proportions of 

 liquid and solid bitumens of which it is composed. It is, of course, the 

 provinee of the chemist to determine these characteristics for all bitu- 

 mens proposed for use in road construction, and to interpret the result 

 in the light of practical experience. 



From the point of view of their solubility or insolubility in petro- 

 leum naphtha the liquid and solid bitumens in use in bituminous high- 

 way construction are composed of two components, one of which has 

 been arbitrarily named as a class petrolenes, soluble in naphtha, and 

 the other, asphaltenes, insoluble in naphtha. The one consists of the 

 liquid and the other of the solid components. Whatever value a bitumen 

 may have as a binding material for highway construction, is due to the 

 presence and the character of the petrolenes. The solid material in 

 itself has no binding power, but by its solution in or mixture with the 

 petrolenes it gives to the latter their binding power, and also adds to 

 their stability. 



The value of a bitumen as a road binder will further depend upon 

 the character of the petrolenes which it contains. If the petrolenes are 

 of a sticky nature, the particular bitumen will be adhesive and ceraenti- 

 tious, whereas if they are merely oily and not sticky, the material will 

 lack in cementitious properties. The asphaltenes impart cohesiveness 

 as distinguished from adhesiveness, and supply body or stability, as has 

 been said, to the binding material. As an example it may be cited that 

 the heavy residuum left on the distillation of paraffine petroleums in the 

 preparation of burning and lubricating oils, consists of practically 100 

 per cent, petrolenes, but these petrolenes are oily and not sticky and 

 adhesive. The same is true of any of the preparations from paraffine 

 petroleums or petroleums containing a considerable amount of hydro- 

 carbons of the paraffine series. It is not in itself a suitable binding 

 material for highway construction as appears, not only for the reasons 

 given, but by its behavior in actual use. The petroleums derived from 

 the asphaltic oils, on the contrary, such as those of Trinidad, Cali- 

 fornia and Mexico, are of a much more sticky character, and are not 

 only in themselves, when reduced to a proper consistency, more suit- 

 able for a binding material, but are particularly desirable when used 

 to soften solid and harder bitumens, known as asphalts, which possess 

 great cohesiveness, but are wanting in cementing properties. The rela- 

 tive proportions of sticky petrolenes and cohesive asphaltenes is the 

 most important element in bitumens which are used in the construction 

 of asphalt pavements and bituminous highways. It has been found 

 that the asphalt cement, that is to say, a solid asphalt combined with a 

 suitable flux to provide the proper consistency for practical use, if it 



