waves; and afterwards become covered with strata of argillaceous, or calcareous earth, andare decomposed. Nor is it more difficult to conceive, how these masses of vegetables may form strata of coal, than that the remains of shells should form the greater part of the globe. The direct proofs which may be given in support of this theory, are, as I have already mentioned, the presence of vegetable and marine substances. The soils which contain coal, are generally of schistus or grit; and as the formation of pyritæ, as well as that of coal, arises from the decomposition of vegetable and animal substances, for sulphur has been proved to exist naturally in certain animals and plants, all pit coal is more or less pyritous; so that we may consider pit. coal as a mixture of pyrites, schistus, and bitumen. The different qualities of coal, arise, therefore, from the difference in the proportions of these principles. The best is that in which the bitumenous principle is the most abundant, and exempt from all impurity.*

It has been conjectured, that the strata of coal keep their course, in general, in a line of bearing quite through a country, and perhaps round, the whole globe. Thus it is asserted, that coals stretch away under the mountains, and emerge

* Chaptal.

M.

emerge again upon the opposite side. But better observation makes it now evident, that coal countries are but patches of limited, though different dimensions; and that coals were among the last formed of the strata of the globe. It was likewise for a long while supposed, that the smell of pit coal was unwholesome. Venel has, however, made many experiments on this subject, and is convinced that neither man nor animal is incommoded by its vapour. Hoffman relates, that disorders of the lungs are unknown in the villages of Germany, where this combustible only is used. And this, indeed, may all be true of coal of a good quality, which does not emit any dangerous vapour; but, when it is pyritous, its smell cannot but be hurtful. How long this fossil may have been known in the world, it is immaterial to inquire. It is given by historians a very modern date, although it is probably considerably older. In the east, in China in particular, there are grounds for believing it was discovered long before it was known in the western world. However this may be, this magazine of combustible matter, which gives men time to repair their exhausted forests, has not been used even in London above 200 years. Nor is it yet admitted even for culinary or domestic purposes

VOL. 11.

H

on

on many parts of the continent, although almost every part abounds in coal mines. Nor though tar is copiously to be extracted from coal, which by the way is another strong proof of its vegetable origin, are we to confound pit coal with charcoal, or what is called carbone in the new French nomenclature, which nevertheless exists ready formed as sulphur, in vegetables and animals.

LETTER

LETTER XXXIX.

WE have seen that water, by its dissolving virtue, and by its fluidity, tends to level the surface of the globe over which it runs; and that it again, by assembling the bodies so carried off, into its bosom, throws off a renovated form, and ultimately produces great and surprizing inequalities. In like manner, beneath the earthy surface of the globe, we shall be able to trace its levelling and its dreadful energy. Below, as strikingly as above, we shall observe the effects of its penetrating and destroying quali ties. So universal has been its devastations, that one might be almost tempted to say with Leibnitz, and in the words of the old philosophy, there is no such thing in matter, as parts permanent and solid.

The conjoint operation of fire and water is tremendous. Fluids, you know, are raised to a boiling state, when the matter of fire passes with such rapidity and force through their substance, as to be superior to the pressure of the

air upon their surface; and when this point is gained, the fire having nothing further to resist it, the heat never rises higher; so that all fluids have a certain fixed degree, at which they boil. Water will not boil (except in some particular cases) but with an heat of 212 degrees. Yet, when the pressure of the atmosphere is almost entirely removed in the vacuum of an air pump, water will boil with an heat not exceeding 95 degrees, or 117 degrees below the heat required in the open air: and hence appears, that fire and air act as antagonists in the operation of boiling.*

it

Water in vapour, occupies 1400 times more space than in fluidity; and by the same degree of heat is rarefied 14,000 times, while air is only rarefied two thirds. Its spring and elasticity, consequently, are such as to produce dreadful explosions when pent up. Even in mechanics, we see it is used to move the heaviest bodies. It favors combustion, and hence Boerhaave looked upon flame to be principally formed of water. At Geyser, in Iceland, says Van Troil, one sees within the circumference of three miles, forty or fifty boiling springs together, which seem to proceed

* Philosophy of the Elements. † Philosoph. Transact.