were followers of Peter John, or Peter Joannis, i.e. Peter the son of John, who flourished in the 12th century. His doctrine was not known till after his death, when his body was taken out of his grave and burnt. His opinions were, that he alone had the knowledge of the true sense wherein the apostles preached the gospel; that the reasonable soul is not the form of man; that there is no grace infused by... PETROLEUM*, or Rock oil; a thick oily substance exuding out of the earth, and collected on the surface of wells in many parts of the world. It is found in some in Italy, and in a deserted mine in the province of Dalame in Sweden. In this last place it collects itself in small hollows of limestone, like resin in the wood of the pine-tree. It is found trickling from the rocks, or issuing from the earth, in many parts of the duchy of Modena, and in various parts of France, Switzerland, Germany, and Scotland, as well as in Asia. It is also found not only on the surface of wells as already mentioned, but mixed with earth and sand; from whence it may be separated by infusion in water. It is of a pungent and acid taste, and smells like the oil of amber, but more agreeable.

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 rendered thinner by distillation with water, and leaves a resinous residuum; when distilled with a volatile alkali, the latter acquires the properties of succinated ammonia, and contains the acid of amber. 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. Monet informs us that some kinds of it are of the density of nut oil. It is insoluble in spirit of wine; which, though it be the greatest solvent 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.

The finer kinds resemble naphtha. Kirwan is of opinion that naphtha is converted into petroleum by a process similar to what takes place in essential oils when exposed to the atmosphere; in which case the oil absorbs not only the pure, but also the phlogisticated part of the atmosphere; in consequence of which several alterations take place in them.

Mr Bouldoc made several experiments with the white petroleum of Modena; an account of which he gave to the Paris academy.

It easily took fire (A) on being brought near a candle, Petroleum, 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 4th 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.

The distinguishing characteristic of the petroleum is its thickness, resembling infiished oil: when pure it is lighter than spirit of wine; but, though ever so well rectified, it becomes in time thick and black as before. 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 so 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.

There are three varieties of it according to Mongez:

1. The yellow, found at Modena in Italy; very light and volatile. 2. The reddish, or yellowish red; some of which is collected at Gabian in Languedoc and in Alsace. 3. The heavy, black, or brown kind, which is the most common, and met with in England, France, Germany, and some other countries. It generally runs out either from chinks or gaps of rocks, or is mixed with the earth, and gushes out of it; or it swims on the water of some fountains, as already mentioned. According to Dr Lippert, a kind of rosin is produced by mixing petroleum with smoking nitrous acid. The taste of this substance is very bitter, but the smell resembles that of musk. The vitriolic acid, according to the same author, produces a resin still more bitter, but without any aromatic smell. Cronfeld enumerates the following species.

I. Maltha, or Barbadoes tar, a thick substance resembling soft pitch. It is found in several parts of Europe and Asia; particularly Sweden, Germany, and Switzerland; on the coast of the Dead Sea in Palestine;

(A) 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 lantern and a candle in it: there were some holes in the lantern, through which the petroleum at a considerable distance sucked out the flame of the candle, and, taking fire, burst up with the noise of a cannon, and tore the man to pieces.