or ROCK-OIL, is an extremely subtile 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 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 Petroleum, down the sides of hills along with little streams of water. It 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.
Mr Boulode made several experiments with the white petroleum of Modena, an account of which he gave to the Paris academy.
It easily took fire on being brought near a candle, and that without immediately touching the flame; and when heated in any vessel, it will attract the flame of a candle, though placed at a great height above the vessel, and the vapour it sends up taking fire, the flame will be communicated to the vessel of heated liquor, and the whole will be consumed. It burns in the water; and when mixed with any liquor swims on the surface of it, even of the highest rectified spirit of wine, which is heavier than pure petroleum. It readily mixes with all the essential oils of vegetables, as oil of lavender, turpentine, and the rest, and seems very much of their nature; nor is this very strange, since the alliance between these bodies is probably nearer than is imagined, as the essential oils of vegetables may have been originally mineral ones, and drawn up out of the earth into the vessels of the plants.
Petroleum, when shaken, yields a few bubbles; but they sooner subside than in almost any other liquor, and the liquor resumes its clear state again almost immediately. This seems owing to the air in this fluid being very equally distributed to all its parts, and the liquor being composed of particles very evenly and nicely arranged.
This extensibility of the oil is also amazing. A drop of it will spread over several feet of water, and in this condition it gives a great variety of colours; that is, the several parts of which this thin film is composed, act as many prisms. The most severe frost never congeals petroleum into ice; and paper wetted with it becomes transparent, as when wetted with oil; but it does not continue so, the paper becoming opaque again in a few minutes, as the oil dries away.
Spirit of wine, which is the great dissolvent of sulphur, has no effect upon petroleum, not even with ever so long a digestion. It will not take fire with the dephlegmated acid spirits; as oil of cloves, and other of the vegetable essential oils do; and in distillation, either by balneum mariae, or in sand, it will neither yield phlegm nor acid spirit; but the oil itself rises in its own form, leaving in the retort only a little matter, thick as honey, and of a brownish colour.
Alonso Barba, in his book of metals, gives a very melancholy instance of the power of petroleum of taking fire at a distance: he tells us, that a certain well, yielding petroleum on the surface of its water, being to be repaired, the workman took down into the well with him a lanthorn and a candle in it; there were some holes in the lanthorn, through which the petroleum at a considerable distance licked out the flame of the candle, and, taking fire, burst up with the noise of a cannon, and tore the man to pieces.
The people of mount Ciaro, in Italy, have some years since found out a much easier way of finding petro-
This mountain abounds with a sort of greyish salt, which lies in large horizontal beds, mingled with strata of clay, and large quantities of a spar of that kind called by the Germans felsiteries; which is the common sort, that ferments with acids, and readily dissolves in them, and calcines in a small fire. They pierce these slates in a perpendicular direction till they find water; and the petroleum which had been dispersed among the cracks of those slates, is then washed out by the water, and brought from all the neighbouring places to the hole or well which they have dug, on the surface of the water of which it swims after eight or ten days. When there is enough of it got together, they lad it from the top of the water with bras basons, and it is then easily separated from what little water is taken up with it. These wells or holes continue to furnish the oil in different quantities for a considerable time; and when they will yield no more, they pierce the slates in some other place.
Petroleum is never used among us; but the French give it internally in hysterick complaints, and to their children for worms: some also give it from 10 to 15 drops in wine, for suppressions of the meninges. This, however, is rather the practice of the common people than of the faculty.