This art was known to and practised by the ancient lapidaries, and it has been revived in recent times.] A fragment of rock pervaded with moisture cracks and splits, if thrown into a furnace, and sometimes with a loud detonation; and it is a familiar observation that the fire, in burning over newly cleared lands, breaks up and sometimes almost pulverizes the stones. This effect is due partly to the unequal expansion of the stone, partly to the action of heat on the water it contains in its pores. The sun, suddenly let in upon rock which had been covered with moist earth for centuries, produces more or less disintegration in the same way, and the stone is also exposed to chemical influences from which it was sheltered before. But in the climate of the United States as well as of the Alps, frost is a still more powerful agent in breaking up mountain masses. The soil that protects the lime and sandstone, the slate and the granite from the influence of the sun, also prevents the water which filters into their crevices and between their strata from freezing in the hardest winters, and the moisture descends, in a liquid form, until it escapes in springs, or passes off by deep subterranean channels. But when the ridges are laid bare, the water of the autumnal rains fills the minutest pores and veins and fissures and lines of separation of the rocks, then suddenly freezes, and bursts asunder huge, and apparently solid blocks of adamantine stone. [Footnote: Palissy had observed the action of frost in disintegrating rock, and he thus describes it, in his essay on the formation of ice: "I know that the stones of the mountains of Ardennes be harder than marble. Nevertheless, the people of that country do not quarry the said stones in winter, for that they be subject to frost; and many times the rocks have been seen to fall without being cut, by means whereof many people have been killed, when the said rocks were thawing." Palissy was ignorant of the expansion of water in freezing--in fact, he supposed that the mechanical force exerted by freezing-water was due to compression, not dilatation--and therefore he ascribes to thawing alone effects resulting not less from congelation. Various forces combine to produce the stone avalanches of the higher Alps, the fall of which is one of the greatest dangers incurred by the adventurous explorers of those regions--the direct action of the sun upon the stone, the expansion of freezing-water, and the loosening of masses of rock by the thawing of the ice which supported them or held them together.]