years, increasiiig with depth in a remarkable manner. In general, there is an additional degree of heat for every 25 or 30 metres, and if this ratio continue, the heat near the centre of the earth must be greater than we can possibly conceive; nor could we otherwise account for the origin of thermal waters, which spring up with the temperature of boiling water.
But if the existence of fire, or at least of excessive heat in the depths of the earth admits of no doubt, it is no less certain that another different and opposite principle exists in its cavities which are often converted into great reservoirs of water. Whether we attribute the origin of springs to precipitations of atmospheric vapour, to the melting of ice, to filtrations of sea water, which by means of capillary tubes may be raised to great distances, or to subterraneous vapours, which in cooling become liquid; the water sometimes erupted from volcanoes, the sudden and terrible inundations of mines, the floods which disappear for ever, the mountains suddenly swallowed up by immense lakes, force us to acknowledge (according to Malte Brun), the existence of great subterranean cavities, full of water. If these volcanoes possess within themselves the power of propagating their fires, and producing dreadful eruptions every time that they are furnished with new food and material, it would appear, on the other hand, that subterraneous water must extinguish, or at least quench and diminish these fires, and occasion the preponderance of the system adopted by the Neptunists, We shall not exclusively embrace either system, but content ourselves with philosophizing in the style of the ancient Academicians, and approving solely of that which appears probable, endeavour to show that volcanic principles in connection with aqueous, are capable of producing the phenomena of earthquakes and eruptions.
If you take, for example, 30 lbs. of iron filings, and 30 lbs. of sulphur, knead them into a paste with water, and bury them in a lump in a trench in the open coimtry, you will shortly see smoke issuing from the spot, with very great heat and convulsion of the ground to some distance, and finally flames.
Now the materials of which volcanoes are composed are pyrites, or sulphurets, that is, sulphur combined with metallic substances,
VOL. II.O
