of carbonate of lime* in solution, is well known; and also that changes of temperature, as well as many other causes, will occasion the calcareous earth to be in part or wholly precipitated. The fur, as it is called, that lines a boiler which has been long in use, affords a familiar illustration of this fact. At the temperature of 60°, lime is soluble in 700 times its weight of water; and if to the solution a small portion of carbonic acid be added, ⠀ a carbonate of lime is formed, and precipitated in an insoluble state. If, however, the carbonic acid be in such quantity as to supersaturate the lime, it is again rendered soluble in water; and it is thus that carbonate of lime, held in solution by an excess of fixed air, not in actual combination with the lime, but contained in the water and acting as a menstruum, is commonly found in all waters. An absorption of carbonic acid, or a loss of that portion which exists in excess, will therefore occasion the calcareous earth to be set free, and precipitated on any substances in the water, such as stones, sprigs, and leaves of trees, &c. Some springs contain so large a proportion of calcareous earth when they first issue from the rocks, and so speedily throw it down in their course, that advantage has been taken of the circumstance to obtain incrustations of various

* Carbonate of lime consists of lime in combination with carbonic acid gas, which is a most abundant natural product. This gas is unrespirable, and when pure, will immediately suffocate an animal immersed in it. It extinguishes flame.

objects, as leaves, branches, baskets, nests with eggs, and even old wigs. The incrusting springs of Derbyshire are celebrated for such productions. These depositions are termed tufa, or travertine; and in Italy, and many other countries, they constitute extensive beds of concretionary limestone, which is often of a crystalline structure. The Cyclo

pean walls and temples of Pæstum, are formed of this substance. At the baths of San Filippo, in Tuscany, where the waters are highly charged with tufa, this property is applied to a very ingenious purpose. The stream is directed against moulds of medallions, and other bas-reliefs, and very beautiful casts are thus obtained; of which we have an example in this medallion, which bears the head of Napoleon, and was presented to me by the Marquis of Northampton.

42. INCRUSTATIONS NOT PETRIFACTIONS.-As specimens of this kind are commonly, but errone

A A A, Extremities of the twig unchanged; B B, the tufaceous crust.

ously, termed petrifactions, I will briefly explain their real nature. We have before us several

incrustations from various places: baskets of shells, and nests with eggs, from Derbyshire; a bird, from Knaresborough, in Yorkshire; and a twig, partially incrusted, from Ireland.

I need scarcely observe, that on breaking such specimens, we find the inclosed substances to have undergone no change but that of decay, in a greater or less degree. In this incrusted bird's nest, the twigs of which it is composed, like the branch above mentioned, are exposed in several places, and, as you perceive, are not permeated by stony matter, but are dry, and brittle. Now, a true petrifaction is altogether of a different nature, the substance being saturated throughout with mineral matter; if we break it, we find that every part of its structure has undergone a change; sometimes flint has filled up the interstices, and upon slicing and polishing it, the most delicate texture of the original may be detected. Wood, for instance, which is so commonly petrified by flint or chalcedony, may be cut so thin, that with a powerful lens the ramifications of the vessels and the structure of the tissues may be seen, and from their form, and disposition, we may determine the particular kind of tree to which the specimen belonged, although it may have been cased up in stone for ages. When bone is petrified, the same phenomena are observable; the most delicate parts of the internal structure are preserved, and all the cells are filled up with stone or spar, which is oftentimes of a different colour from that of the

walls of the cells, and thus a natural anatomical preparation, of great beauty and interest is formed.

43. LAKE OF THE SOLFATARA.-The celebrated lake of the Solfatara lies in the Campagna between Rome and Tivoli, and is fed by a stream of thermal water which flows into it from a neighbouring pool. The water is of a high temperature, and is saturated with carbonic acid gas, which, as the water cools, is constantly escaping, and keeping up an ebullition on the surface. The formation of travertine is so rapid, that not only the vegetables and shell-fish are surrounded and destroyed by the calcareous deposition, but insects also are frequently incrusted. In these beautiful specimens of travertine from Solfatara, vegetable impressions are distinctly seen, the cavities in the mass having been occasioned by the decomposition of the vegetable matter.* The stream that flows out of the lake fills a canal, which is conspicuous at a distance, from the line of vapour emanating from the water.

A considerable number of the edifices of both ancient and modern Rome, are constructed of travertine, derived from the quarries of Ponte Luccano, which have clearly originated from a lake of the same kind. Pæstum is also built of calcareous tufa, derived from similar deposits. "The waters of these lakes," says Sir Humphry Davy, "have their rise at the foot of the Apennines, and hold in solution carbonic acid, which has dissolved a portion of See Appendix B.

the calcareous rocks through which it has passed; the carbonic acid is dissipated by the atmosphere, and the marble, slowly precipitated, assumes a crystalline form, and produces coherent stones. The acid originates in the action of volcanic fires on the calcareous rocks of which the Apennines are composed, and carbonic acid being thus evolved, rises to the source of the springs derived from the action of the atmosphere, gives them their impregnation, and enables them to dissolve calcareous matter."

44. MARBLE OF TABREEZ.-In Persia, a beautiful transparent limestone, called Tabreez marble, is formed by deposition from a celebrated spring near Maragha, where the whole process of its formation may be seen. In one part the water is clear, in another dark, muddy, and stagnant; in a third it is very thick, and almost black; while in the last stage it is of a snowy whiteness. The petrifying pools look like frozen water: a stone thrown on them breaks the crust, and the water exudes through the opening; and in some states the process has proceeded so far as to admit of walking on the surface of the lake. A section of the stony mass resembles an accumulation of sheets of paper, being finely laminated; and such is the tendency of this water to solidify, that the very bubbles on its surface become hard, as if they had been suddenly arrested, and metamorphosed into stone.*

Morier's Travels.