CHEMICAL COMPOSITION. Asphalt is not a mineral of definite chemical composition. Generally, it is a compound con sisting of various hydro-carbons, which can be separated from each other only with great difficulty. Its chemical composition when pure is: carbon, 80 to 88 per cent; oxygen, 0.5 to 10 per cent; hydrogen, 9 to 11 per cent; nitrogen, 0 to 1 per cent. Asphalt is often found mixed with other minerals, which may be called im purities, the most common of these being sulphur, which sometimes constitutes from 1 to 10 per cent of the whole. Varieties of asphalt from different localities seldom, if ever, agree with each other in chemical composition.
Asphalt may be separated, with more or less readiness, into several different substances which differ somewhat in chemical composition and widely in physical properties, the presence or ab sence of which has an important influence on the value of the asphalt for paving purposes. The principal of these substances are asphaltine, petroline, and retine. Under the head of asphaltine has been classed that part of the asphalt which is soluble in chloroform and bisulphide of carbon, and not in ether or naphtha; under the head of petroline has been classed that part which is soluble in ether and naphtha; and under retine has been classed that part which is soluble in alcohol. Asphaltine is hard and brittle, requires a high heat to melt it, or burns without becoming melted, and has very little, if any, of the adhesive qualities which make asphalt useful as a cement. Petroline is softer than asphaltine, becomes fluid at a lower temperature, has great adhesive or cementitious qualities, and is the valuable part of asphalt for most industrial work. Retine
possesses the character of vegetable resin, and is not considered as adding anything to the value of asphalt for paving purposes.
It is very questionable, however, whether this division is well founded. Chemists differ widely as to the quantities of each of these so-called elements found in samples from the same locality, the results varying with the details of the methods of making the determinations, and with slight variations in the solvents employed.
Recently investigators are inclined to class all the components of asphalt under two heads only, the active and the inert. The active element is that part which is easily melted by heat, is readily solu ble in ether or naphtha, and is highly adhesive and cementitious; while the inert material is the hard and brittle kart which is not readily melted by heat, and which adds nothing to the cementitious properties of the asphalt. The ratio in which the active and the inert constituents are combined is the true index of the value of the asphalt for use as a cement; but for other industrial purposes, such as insulating electric wires, etc., it has no practical value, for the active and the inert constituents appear to be equally good as non conductors.
Asphalts from different sources should not be compared by their chemical analyses unless exactly the same methods and solvents have been used in each case. Further, it has been found that the chemical analysis of asphalt is not a reliable indication of its value as a cement, since asphalts having practically the same composition differ comparatively widely as to their physical properties.