also called rock-oil, is an extremely subtle and penetrating fluid, and is much the thinnest of all the native bitumens. It is very light and very pellucid; but though equally bright and clear under all circumstances, it is liable to a very great variety in its colour. It is naturally almost colourless; and in its appearance greatly resembles the most pure oil of turpentine; this is called white petroleum, though it has no more colour than water; it is sometimes tinged of a brownish reddish, yellowish, or faint greenish colour; but its most frequent colour is a mixture of the reddish and blackish, in such a degree that it looks black when viewed behind the light, but purple when placed between the eye and a candle or window. It is of a pungent and acrid taste, and of a very strong and penetrating smell, which very much approaches to that of the distilled oil of amber. The white PHA

is most esteemed. It is so very inflammable, that while it floats on the surface of the water, as it does in many parts of Italy, it takes fire at the approach of a candle.

Petroleum is found in rivers, in wells, and trickling down the sides of hills along with little streams of water. In short, it is the most frequent of all the liquid bitumens, and is perhaps the most valuable of them all in medicine. It is to be chosen the purest, lightest, and most pellucid that can be had, such as is of the most penetrating smell and is most inflammable.

It is principally used externally, in paralytic cases, and in pains of the limbs.