4o6 JOSEPH STRUTHERS 



material, the treatment has for its object the removal of water, 

 volatile hydrocarbons, and mechanically suspended mineral 

 and vegetable matter, but with asphaltums to be used in 

 pavement construction it is essential to allow the mineral mat- 

 ter to remain in the product, for the reason that the asphaltum 

 in this case is to serve solely as a binding and waterproofing 

 material. In fact, asphaltic rock of the proper composition 

 is not refined before use, but merely crushed, heated and laid 

 in place. The simplest method of extracting asphaltum from 

 its compounds is by boiling in water, which causes the lighter 

 asphaltum to rise to the surface, from whence it can be easil}' 

 removed. Common salt or calcium chloride may be substi- 

 tuted for water in the treatment of material requiring a higher 

 temperature or a heavier supporting liquid. In other cases 

 asphaltum is extracted by means of a solvent, such as carbon 

 bisulphide or naphtha, and obtained from the solution by 

 heating in stills, which causes the volatile solvent to be dis- 

 tilled for repeated use and leaves the liquid asphaltum residue 

 in a practically pure condition, 



