210 



THE WORLD. 



mineral substances; or with some gas. The presence of carbon- 

 ate of lime, or lirne in combination with carbonic acid, is easily 

 shown by the calcareous lining or incrustation of a tea-kettle, or 

 a boiler which has been sometime in use. Some springs contain 

 so large a quantity of calcareous matter that they throw it down 

 as they flow along, incrusting various objects which are placed in 

 them. The springs of Derbyshire England, are particularly re- 

 markable for this, and incrustations of leaves, branches, bas- 

 kets &c., are easily procured. At the baths of San Fillippo, in 

 Tuscany, where the waters are highly charged with carbonate and 

 sulphate of lime ; medallions are formed by first directing the 

 water to a cistern where the sulphate of lime, (gypsum) is do- , 

 posited. It is then conveyed to a chamber through a tube, from 

 the end of which it falls ten or twelve feet, the current being bro- 

 ken by numerous small sticks crossing each other, by which 

 means the spray is dispersed about the room. The Vnoulds of the 

 medallions are placed underneath, rubbed over with a little soap, 

 and the water striking upon them leaves particles of carbonate 

 of lime, which, gradually increasing, finally gives an exact and 

 beautiful white crust. So rapid is the deposition of earthy mat- 

 ter by these springs, that a stratum of stone a foot thick is annu- 

 ally deposited, and is employed for building purposes. The hill 

 of San Vignorn, in Tusca~ny, a few miles from San Fillippo, has 

 a thermal spring upon its summit, and from this opening, a deposit 

 of travertine, or concretionary limestone has been formed two 

 hundred feet thick, and of great hardness. We must be careful 

 and not confound these incrustations with true petrefactions. In 

 the one case, as for example an incrusted twig, the inclosed sub- 



stance will be found to have undergone no alteration, but that of 

 natural decay, but a'true petrefaction, is saturated throughout with 



