BITUMEN, in chemistry. The term bitumen has often been applied by che mists to all the inflammable substances that occur in the earth ; but this use of the word is now so far limited, that sul phur and millite are most commonly ex cluded. It would be proper to exclude amber likewise, and to apply the term to those fossil bodies only which have a cer tain resemblance to oily and resinous substances. Bituminous substances may be subdivided into two classes, namely, bituminous oils, and bitumens, properly so called. The first set possesses nearly the properties of volatile oils, and ought, in strict propriety, to be classed with these bodies; but as the chemical pro perties of bitumens have not yet been in vestigated with much precision, it is deemed rather premature to separate them from each other. The second set possess properties peculiar to themselves. Only two species of bituminous oils have been hitherto examined by chemists. Others indeed have been mentioned; but their existence has not been sufficiently authenticated. These two species are called petroleum, and maltha, or sea wax ; the first is liquid, the second solid. See PETROLEUM and MALTHA.
The true bituminous substances may be distinguished by the following proper ties :—They are either solid, or of the consistence of tar : their colour is usually brown or black : they have a peculiar smell, or at least acquire it when rubbed ; this smell is known by the name of the bituminous odour ; they become electric by friction, though not insulated; they melt when heated, and burn with a strong smell, a bright flame, and much smoke : they are insoluble in water and alcohol, but dissolve most commonly in ether, and in the fixed and volatile oils; they do not dissolve in alkaline leys, nor form soap ; acids have little action on them ; the sulphuric scarcely any ; the nitric, by long and repeated digestion, dissolves them, and converts them into a yellow substance, soluble both in water and alco hol, and similar to the product formed by the action of nitrous acid on resins. The
bitumens at present known may be re duced to three ; namely, asphaltum, mi neral tar, and mineral caoutchouc. Bitu men has been found also united to a re sinous compound, in a curious substance first accurately examined by Mr. Hatch ett, to which he has given the name of retinasphaltum. United to charcoal in various proportions, it constitutes the nu merous varieties of pit-coal, so much em ployed in this country as fuel. The as phaltum found in Albania is supposed to have constituted the chief ingredient of the Greek fire. Asphaltum is seldom ab solutely pure ; for when alcohol is digest ed on it, the colour of the liquid becomes yellow, and by gentle evaporation a por tion of petroleum is separated. Mineral tar seems to be nothing else than asphal tum containing a still greater proportion of petroleum. When alcohol is digested on it, a considerable quantity of that oil is taken up ; but there remains a black fluid substance like melted pitch, not acted upon by alcohol, and which there fore appears to possess the properties of asphaltum, with the exception of not be ing solid. By exposure to the air, it is said gradually to assume the state of as phaltum.