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fies with astonishing rapidity. To form it into mill-ftones, no other opera tion is neceffary but, after having cleared away about two feet of foil, to kneed the under-ftratum and form it into the fhape required, and then cover it loofely again with the fame foil. In a year's time the mill-stone is found perfectly hardened. The hardness of coble and pebble stones occa fions them to be generally looked upon as ftones of very antient petrification: this is, however, often not the cafe. In the dried bed of a torrent in Italy, I remember to have gathered in a short time, as I walked along it, a number of fpecimens of pebbles of very different degrees of hardness and formation— from that which still received the impreffion of the finger, or easily crumbled when pressed, to that degree which furpaffes the hardness of most other kinds of ftones. These new-moulded pebbles, frequently interfected by veins of different colours, perfectly resembled others in the completeft degree of induration. Clays, thus rounded and gradually hardened, are easily to be found in the bed of every torrent exposed to be fometimes filled with waters, and at other times laid dry. The petrifying qualities of many waters are too well known to be infifted upon. One inftance of the abundance and minuteness of ftony particles contained in waters, and of the celerity with which they fuperinduce a ftony incruftation, is worthy to be recorded. It is mentioned by Mr. Dietrich, the late philofophic mayor of Strasburg, and inferted by Mr. de Buffon in his Natural History of Minerals. By the ingenious management of the waters of Santo Filppo on the mountain Santa Fiora near Sienna, Doctor Leonardo Vegni has contrived, to take off the impreffion of medals and baffi relievi, by incruftation; even casts have been taken from buftos, and he hopes at length to be able to take off whole statues. Any porous fubftances may be impregnated both inwardly and outwardly with thefe fediments; and if the animal or vegetable substance which ferves as a mould fhould decay, the remaining concretion will appear a true petrification of the body itself, though it has really been only incrusted, interiorly as well as exteriorly. If woods or fhells are thrown into these quickly-accumulating concretions, they will be as completely petrified within them, as we fometimes find fuch fub

ftances

ftances in fofil bodies. By a diffolution of fernambuco wood, the doctor gives various colours to thefe petrifications. Thus thefe great interpreters of nature cannot help acknowledging the extreme promptitude of her actual proceffes on a small fcale, whilft they difavow the poffibility of a like celerity in her primæval powers of concretion on a larger; because it would militate against their favourite fyftem, of flow operations during an infinity of ages.

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It is well known that artificial ftones, perfectly refembling in appearance and folidity Portland stone, have been long fabricated in London. Variegated alabasters, jafpers, and agates, are cryftallized at Ashbourne and other places. The art of man has not yet been able to give their genuine luftre or folidity to the imitations of diamonds and the finer precious stones; but all other kinds of stones are imitated to a degree of perfect deception by a hundred artifts in every part of Europe. Shall plaftic Nature in her fulleft vigour, the goddefs whom fome philofophers alone adore, or the omnipotent God of Nature herself, require thousands and ten thousands of years to complete the originals, however greatly they may furpass the imitations?

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The crystallization, or what is called the vegetation, of lead in water by zinc is but lately difcovered. Sugar of lead in water is fo powerfully and quickly attracted by a piece of zinc, that in a few hours the whole forms a beautiful arborization pendent in the bottle from that matter. Such crystallizations are also effected by fire. Mr. Ferber observes, that various shorl crystallizations are formed in the empty bubbles of the torrents of lava, cooling on the fuperficies whilft yet in violent fufion underneath. Diamond-like crystallizations are likewise formed in it, though much inferior in hardness, in regularity, and brightness of colours, to real gems which are cryftallized in water. When the whole earth was yet a liquid pulp 3 H 2 fwimming

fwimming in every kind of menftruum, actuated upon by the great mass of univerfal light, and when dry land appeared, by the full force of its ruling. fun, how can we fuppofe that these fame laws of nature in their fullest vigour fhould require thousands of years to cryftallize or coagulate the various fubftances of which we fee it now compofed? What these watery menftrua did not effect, frequent volcanic fires-natural in the first great fermentation of matter, either immediately after the creation or after the deluge, when its exterior coat was nearly reduced to the fame fituation— might well bring to perfection during the first four centuries, when men were yet confined to a narrow spot. That at this laft epoch its whole exterior furface was convulfed, diflocated, and diffolved, every monument of nature amply teftifies. God had faid that he would deftroy not only the whole impious race of men but the earth with them; and he afterwards promises Noah that there shall not be another deluge-" diffipating," according to the literal words of the Hebrew; according to the Samaritan verfion "fcattering," or "difperfing;" tranflated by the Greek" to cor"rupt," or "disfigure," and by the Syriac and Arabic " to deftroy" the earth; that is to fay, fince the body of the globe remained, its outward form and furface.

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SIR,

ON THE

STRUCTURE

OF THIS GLOBE.

LETTER V.

Various Opinions on the Nature of Light, Heat, and Fire..

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11

AFTER having in my preceding letter laid before you the celebrated fyftem of Mr. de Buffon, and the feveral opinions of many other modern philofophers tending with him to prove not only the high antiquity of this globe, but the numberlefs ages required to elaborate and confolidate its component parts, and to fashion its actual ftructure; and having given you with freedom my reafons for diffenting from their pretended irrefragable proofs of this thefis; you will give me leave, previous to any further difcuffion on that fubject, to fay, with extreme diffidence, a few words on the nature of Light, Heat, and Fire. On this head natural philosophy, as yet in its infancy, is very undecided, and has produced a number

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ber of difcordant opinions. It is, however, a point which appears to me of great importance, and by no means indifferent for the fettling of our ideas, both on the original and actual formation of this earth and on the continued operations of nature. On this difficult fubject, I shall rather produce the different opinions of others than presume to offer any fixed fentiment of my own. Heat, Fire, and Light, have till very lately been generally looked upon as one and the fame matter, proceeding from one and the fame cause, distinct only in degree-fire generated from heat, and light from fire. All three, however, appear in many instances perfectly distinct, and discriminated by various and frequently oppofite effects. The fource of all three has been generally placed in the fun, or at least that body has been looked upon as their great magazine; but on this too there are different opinions.

It is on the basis of fire firft exifting in the fun and purloined from thence by other bodies that Mr. de Buffon has laid his fyftem. Having once kindled and fixed it there as in its feat, he imagines whatever relics remain of it in our planet to be no other than a flackened portion thence originally derived. As we have already seen, he confiders not heat, fire, and light, deemed only different degrees from one and the fame cause, as a distinct element from his fole vitreous fubftance. They are, according to him, purely generated from, and the effects of, preffure and friction. By their operation, without the intervention of any other distinct matter, the fun

was

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