THE CORROSION OF BUILDING STOXES. 281 



The travertine used as a building stone is chiefly derived from the 

 quarries of Ponte Lucano, and is the deposit that was formed on the 

 bed of a lake like that of Tartarus. The celebrated cascade of the 

 Anio at Tivoli forms calcareous stalactites, and all the country round 

 has rivulets, caverns, and deposits where this formation may be seen 

 in progress or completed. 



It varies considerably in structure : some specimens are compact 

 and smooth, others have the appearance of a petrified moss, and great 

 varieties may be found among the materials of a single building. It 

 is, however, usually rough and more or less spongy-looking, as above 

 stated, but this structure does not seem to affect its stability, at least, 

 not in the climate of Italy. Whether it would stand long frosts is an 

 open question. The night frosts at and about Home are rather 

 severe, but usually followed by a warm sunny day ; thus there is no 

 great penetration of ice. 



Every specimen I have examined shows a remarkable compactness 

 of molecular structure in spite of visible porosity. All give out a clear 

 metallic ring when struck, and the intimate surface, if I may so de- 

 scribe the surface of the worm-like structure it sometimes displays, 

 is always clear and smooth as though varnished. To this I attribute 

 its durability. Lest the above description should appear self-contra- 

 dictory, I will explain a little further. If melted glass were run into 

 threads, and those threads while soft were allowed to agglomerate 

 loosely into a convoluted mass, it would, as regarded in mass, have a 

 porous or spongy-looking structure, but nevertheless its molecular 

 structure would be compact and vitreous ; there would be mechani- 

 cal, but not molecular, porosity. Travertine is similar. 



Have we any travertine in England ? This is a practical question 

 of some importance, and one to which I have no hesitation in reply- 

 ing, Yes. There is plenty formed and forming in the neighborhood 

 of Matlock, but that which I have seen on the face of caverns, etc. is 

 not so compact and metal-like as the Italian. This, however, does 

 not prove the entire absence of the useful travertine. Not having 

 any commercial interest in the search, I have only looked at what has 

 come in my way, but have little doubt that there are other kinds be- 

 sides those I saw. I have also seen travertine in course of formation 

 in Ireland, where I think there is a fine field for exploration in the 

 mountain limestone regions, which have been disturbed by volcanic 

 action of the Miocene period. The travertines of Italy are found in 

 the neighborhood of extinct volcanoes. 



The classic associations of this material, its remarkable stability, 

 and the facility with which it may be worked, render it worthy of 

 more attention than it has yet received from British builders. 



CHAPTEE XLI. 



THE CORKOSION OF BUTLDING STONES. 



ABOUT fifty years ago two eminent French chemists visited London, 

 and rather "astonished the natives" by a curious feature of their 

 dress. They wore on their hats large patches of colored paper. 



