These oils are obtained by distillation from Scotch shale, a black, somewhat flaky mass of homogeneous structure, with an irregular, glossy surface. It is usually soft enough to be cut, and when ignited burns with a flame. The better qualities yield from thirty to thirty-three per cent of oil; the poorer grades are stony and slate-like and produce little oil.

When submitted to distillation in large vertical stills, an uncondensable gas and water containing a large proportion of ammonia first come over; then a crude oil of a thickish dark reddish brown, with a peculiar odor and of a specific gravity of from 0.865 to 0.890, or 32° to 27° Baume, is obtained. This is again subjected to distillation, and naphtha and light-colored oil, which gradually becomes darker, are obtained. Solid paraffine begins to show itself, followed by a thick, brown oil, and finally coke is left in the still.

The oil is treated with sulphuric acid and with a solution of caustic soda. When again distilled the oil yields a light spirit known as green naphtha, a light oil known as "second run oil," and a dark-colored, heavy oil, known as "green oil," and coke is left as residuum in the still.

The naphtha is treated with sulphuric acid and solution of caustic soda, and is separated into several grades of illuminating oil.

The "green oil" is subjected to a freezing temperature, and paraffine wax is obtained from it by pressure. The solid paraffine wax obtained is about twelve to fifteen per cent. The remaining oil is known as "paraffine oil," and was formerly used in the manufacture of lubricating oils.

The specific gravity of the light oils from shale is about 0.730 to 0.760; that of the heavier oils is about .810 to .820. The oil from the paraffine pressings has a specific gravity of about 0.872 or about 30° Baume.

Shale oil is treated with a small proportion of sulphuric acid and caustic soda, about one per cent, and redistilled, yielding the following grades of illuminating oils:

No. 1, best, with a specific gravity of 0.800 and 100° F. flash point.

No. 2, first quality, with a specific gravity of 0.810 and 103° F., flash point.

No. 3, second quality, with a specific gravity of 0.815 and 120° flash point, 4

No. 4, lighthouse oil, with a specific gravity of 0.820 and 160° F., flash point.

No. 5, marine sperm oil, with a specific gravity of 0.830 and 230° F., flash point.

A fuel oil of about 0.840 specific gravity is left.

Steam is used in the still and controls the distillation ; it lowers the temperature of the distillate and facilitates the passing of the vapors from the still.

The mineral oils have been entirely superseded by the petroleum oils since the discovery and utilization of the latter.