BUILDING AND ORNAMENTAL STONES. 335 



backings would seem to show that if not absolutely fire-proof, they are 

 very nearly so.* 



It must be remembered, however, that the sudden cooling of the 

 surface of a heated stone, caused by repeated dashes of cold water, has 

 often more to do with its disintegration than heat alone. 



Effects of friction. — The amount of actual wear to which stones in the 

 walls of a building are subjected is naturally but slight in comparison 

 with those in the sills, steps, and walks, which are subject to the fric- 

 tion of feet and other agencies. Nevertheless it is sufficient in many 

 cases to become appreciable after the lapse of several years. The 

 striking effect produced by wind-blown sands in the Vv^estern States 

 and Territories has often been alluded tot and even in the Eastern 

 States, as at Cape Cod, Massachusetts, there may frequently be seen 

 window-panes so abraded by blowing sand as to be no longer trans- 

 parent.! 



This same abrading process is going on in all city streets, where the 

 wind blows dust and sand sharply against the faces of the buildings; 

 not with sufficient force, it may be, to perceptibly wear away the fresh 

 stone, but yet forcibly enough to crumble away the small particles 

 already loosened by atmospheric decomposition and thus expose new 

 surfaces to be acted upon. Professor Egleston§ states that in many of 

 the church-yards of New York City the effects of this abrasive action 

 can be seen where the stones face in the direction of the prevailing- 

 winds. In such cases the stones are sometimes worn very nearly smooth 

 and are quite illegible from this cause alone. 



Effects of growing organisms. — It is in such exposed situations, as 

 above mentioned, that a stone is often protected from serious loss by a 

 coating of lichens or mosses, which by growing over its surface shield 

 it from the abrasive action. The full effect of growing organisms upon 

 the surface of stones is still, however, a matter of dispute. By some 

 authorities || it is thought that they give rise to small amounts of organic 

 acids which exercise a corrosive influence. By others they are con- 

 sidered as beneficial, since they protect the stone from the sun's rays 

 and the rain and wind. It seems probable that they may exert either 

 a harmful or beneficial action according to the kind of stone on which 



* Cutting's experiments (Weekly Underwriter) showed that up to the point at 

 which they are converted into quicklime (that of bright redness), limestones are less 

 injured by heat than either granite or sandstones, a result not fully borne out by the 

 experiments of Winchell (Geol. of Minn., Vol. I, p. 197-201). 



t On the Grooving and Polishing of Hard Kocks and Minerals by Dry Sand. W. 

 P. Blake. Proc. A. A. A. S., Providence meeting. 



% There is on exhibition in the National Museum a plate of glass formerly a window 

 in the light-house at Nauset Beach, Massachusetts, that was so abraded by wind- 

 blown sand during a storm of not above forty-eight hours' duration as to be no longer 

 serviceable. The grinding is as complete over j:]je entire surface as though done by 

 artificial means. 



§ Am. Arch., September 5, 1885, p. 13. 



|| See Winchell, Geol. of Minn,, Vol. i, p. 188, 



