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Oh. XVIL] 



CALCAREOUS PRECIPITATES. 



405 



»as with, a very small quantity of sulphuretted hydrogen. 



Its high temperature, which is pretty constant at 80° of Fahr., 

 and the quantity of carbonic acid that it contains, render it 

 peculiarly fitted to afford nourishment to vegetable life. The 

 banks of travertin are everywhere covered with reeds, lichen, 

 confervse, and various kinds of aquatic vegetables ; and at the 

 same time that the process of vegetable life is going on, the 

 crystallisations of the calcareous matter, which is everywhere 

 deposited, in consequence of the escape of carbonic acid, 

 likewise proceed. — There is, I believe, no place in the world 

 where there is a more striking example of the opposition or 

 contrast of the laws of animate and inanimate nature, of the 

 forces of inorganic chemical affinity, and those of the powers 



of life. 5 * 



The same observer informs us that he- fixed a stick in a 



mass of travertin covered by the water in the month of May, 

 and in April following he had some difficulty in breaking, 

 with a sharp-pointed hammer, the mass which adhered to 

 the stick, and which was several inches in thickness. The 

 upper part was a mixture of light tufa and the leaves of con- 

 ferva; below this was a darker and more solid travertin, 

 containing black and decomposed masses of confervas ; in the 

 inferior part the travertin was more solid, and of a grey 

 colour, but with cavities probably produced by the decompo- 

 sition of vegetable matter, f 



The stream which flows out of this lake fills a canal about 

 nine feet broad and four deep, and is conspicuous in the 

 landscape by a line of vapour which rises from it. It de- 

 posits calcareous tufa in this channel, and the Tiber probably 

 receives from it, as well as from numerous other streams, 

 much carbonate of lime in solution, which may contribute to 

 the rapid growth of its delta. A large proportion of the 

 most splendid edifices of ancient and modern Rome are built 

 of travertin, derived from the quarries of Ponte Lucano, 

 where there has evidently been a lake at a remote period, on 

 the same plain as that already described. 



Travertin of 



— In the same neighbourhood the cal- 



careous waters of the Anio incrust the reeds which grow on 



* Consolations in Travel, pp. 123-125. 



f Ibid. p. 127. 



