62 Proceedings of the Royal Society of Edinburgh. [Sess. 
journey have left behind their more sluggish molecules in proportion to 
the distance travelled. The strata travelled through are very largely close- 
grained calcareous shales. 
It will be seen later that similar filtration can be conducted experimentally. 
Mention has been made above of the inspissation or drying up of natural 
petroleum. This process is probably somewhat complicated : it is accom- 
panied by a certain amount of oxidation, and probably also by the forma- 
tion of unsaturated hydrocarbons, but for all practical purposes we may 
look upon it as the loss by evaporation of the lighter constituents in the 
complicated mixture of hydrocarbons which is known as crude petroleum. 
Thus the butane and probane and such light saturated hydrocarbons are 
lost, and the residue becomes gradually richer and richer in the heavier 
members of the series, and more especially in the unsaturated hydrocarbons. 
At the same time, as will be seen later, the more highly inspissated a 
petroleum becomes, the richer it will be found in compounds containing 
oxygen, sulphur, and nitrogen. 
The next phenomenon of oil-fields to be considered is the occurrence of 
intrusive solid bitumens or manjaks. The term manjak is used as the oldest 
term (dating from the seventeenth century) to include the whole series 
of minerals to which local names have been given in many parts of the 
world. Uintaite, grahamite, gilsonite, manjak, glance pitch, wurtzelite, and 
albertite are some of the names used, and they are classified according to 
their percentages of fixed carbon and their behaviour under the action of 
certain solvents. 
The following table gives the characteristics of a number of these 
manjaks : — 
Mineral. 
Specific 
Gravity. 
Bitumen, 
Percentage 
soluble 
in CS 2 . 
Malthenes, 
Percentage 
soluble in 
Petrol. 
Fixed 
Carbon. 
Gilsonite . . . 
1-04 
93-4 to 99 5 
35 to 72 
3-3 to 26-2 
Barbados manjak 
1*08 
97-4 to 99-2 
15 to 36 
25 
Egyptian glance pitch 
1-09 
99'7 
23-5 
15 
Trinidad manjak 
1*09 to IT 
84 to 96*2 
6-3 to 56 
24 to 33 
Grahamite 
1T6 
94T to 98-2 
•4 to 3*3 
41 to 53 
Wurtzelite 
1-05 
6-7 to 12-8 
5-2 to 8-8 
Albertite .... 
1*07 to 1-2 
1*6 to 11*9 
Trace to 32 
29-8 to 54-2 
It will be seen that there is a great deal of variation in the constants. 
This is due not only to there being different percentages of inorganic 
