280 SCIENCE IN SHORT CHAPTEES. 



now only indistinctly visible here and there, and if I remember 

 rightly, some have disputed its existence. 



But this travertine, though so familiar to the Italian, is such a 

 rarity here that some further description of its structure and compo- 

 sition may be demanded. It is a limestone formed by chemical pre- 

 cipitation. Most limestones are more or less of organic origin, are 

 agglomerations of shells, corals, etc., but this is formed by the same 

 kind of action as that which produces the stalactites in limestone 

 caverns. It has some resemblance to the incrustation formed on 

 boilers by calcareous water. Although the material of so many 

 ancient edifices, it is, geologically speaking, the youngest of all the 

 hard rocks. Its formation is now in progress at some of the very 

 quarries that supplied Imperial Borne. 



On the Campagna, between Eonie and Tivoli, is a small circular 

 lake, from which a stream of tepid water, that wells up from below, 

 is continually flowing. Its local name is " The Lake of Tartarus." 

 The water, like that of Zoedone, or soda-water or champagne, is 

 supersaturated with carbonic acid that was forced into it while under 

 pressure down below. This carbonic acid has dissolved some of the 

 limestones through which the subterranean water passes, and when 

 it comes to the surface, the carbonic acid flies away like that which 

 escapes when we uncork a bottle of soda-water, though less suddenly, 

 and the lime losing its solvent is precipitated, and forms a crust on 

 whatever is covered by the water. 



When I visited this lake in the month of February it was sur- 

 rounded by a chevaux de frise of an extraordinary character ; thou- 

 sands of tubes of about half an inch to one inch in diameter outside, 

 with calcareous walls about one eighth of an inch in thickness. These 

 were standing up from two to three feet high, and so close together 

 that we had to break our way through the dense palisade they formed 

 in order to reach the margin of the lake. After some consideration 

 and inquiry, their origin was discovered. They are the incrusted 

 remains of bullrushes that had flourished in the summer and died 

 down since. During the time of their growth the water had risen, 

 and thus they became coated with a crust of compact travertine. 

 This deposition takes place so rapidly that a piece of lace left in the 

 rake for a few hours comes out quite stiff, every thread being coated 

 with limestone. Such specimens, and twigs similarly covered, are 

 wold to tourists or prepared by them if they have time to stop. Sir 

 Humphry Davy drove a stick into the bottom of the lake and left it 

 standing upright in the water from May to the following April, and 

 then had some difficulty in breaking with a sharp-pointed hammer 

 the crust formed round the stick. This crust was several inches in 

 thickness. That which I saw round the ex-bulrushes may have all 

 been formed in a few days or weeks. The rivulet that flows from the 

 lake deposits travertine throughout its course, and when it overflows 

 leaves every blade of grass that it covers incrusted with this limestone. 



Near to the Lake of Tartarus is the tiolfatara lake which contains 

 similar calcareous water, but strongly impregnated with sulphuretted 

 hydrogen ; it consequently deposits a mixture of carbonate and sul- 

 phide of calcium, a sort of porous tufa, some of it so porous that it 

 floats like a stony scum, forming what the cicerone call " floating 

 islands." Lyell, in his "Principles of Geology," confounds these 

 lakes, and describes Tartarus under the name of Solfatara. 



