276 ECONOMIC GEOLOGY 
the camphenes, C,H,, _ 4 (e.g. CoHy) ; the benzenes,! Chon _g 
(e.g. CgHy), and the naphthenes, ChHypng + Hg (e.g. CgH, 
C,Hg). The series include many hundreds of different com- 
pounds. 
Puysicar CrassiricaTion—Petroleum is divided into four 
groups on physical properties. The first group includes 
marsh gas and natural gas, which are gases at ordinary 
temperatures, and the volatile oils which evaporate below 
300° F. such as the petrol ethers, e.g. cymogene that vaporises 
at 82° F., and the petrol spirits, such as petrol or gasolene 
as it is known in America. Naphtha and benzine are grades 
of petrol. The second group includes the lighting oils, e.g. 
kerosene, which distil at temperatures of from 300° F. to 
570° F. The third group consists of the heavy lubricating 
oils which are not inflammable and distil at above 570° F. 
The fourth group includes the solid petroleums, such as 
paraffin wax, ozokerite, and such bitumens as albertite, 
grahamite, elaterite, etc. 
Baum# Scare—Specific gravity gives useful guidance as to 
the nature of a crude oil. The light oils contain more petrol 
than those of a higher specific gravity. Oils are usually 
compared by the Baumé scale, which states the degree to 
which an hydrometer (graduated from 10 to 100) will sink 
in the oil. The specific gravity of water is marked on the 
scale as 10. The lighter the oil the deeper the hydrometer 
sinks, so that the Baumé degree is higher. Thus 20°B 
marks a specific gravity of -9333, and 30° B. of ‘875. This 
scale directly indicates the weight of the oil; for a fluid 
of 10° B. weighs 10 1b. to the gallon; one of 20° B., sp. gr. 
-933, weighs 0-33 pounds to the gallon; one of 30°B., 
sp. gr. +875, weighs 8:75 1b. to the gallon, and so on. This 
convenient relation does not apply to the American gallon, 
which weighs 8-33 Ib. instead of 10 Ib. to the gallon. 
SuppLy oF OrL—In the search for oil geological guidance 
is now generally recognized as indispensable. J. E. Pogue 
in his Economics of Petroleum (1921, p. 343), reports that 
according to the records of a great American oil company, 
8c per cent. of the wells sunk on geological advice had been 
1 Benzene, CyH, must be distinguished from benzine, a grade of 
petrol.