BUILDING AND ORNAMENTAL STONES. 335 



l)iU'kiiii;s would >seein to show lliat il' not absolutely liic proui', tliey are 

 very nearly so.* 



It must be vemeuibered, bowever, 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. 



Ejects 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 fi ic 

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

 cases to beconu^ appreciable after the lapse of several years. The 

 striking effect produced by wind blown sands in the Western States 

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

 States, as at Cape Cod, Massachr.setts, there may fre(iuently be seen^ 

 Avindow-panes so abr;jded by blowing sand as to be no longer trans- 

 pa rent.:j: 



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

 wind blows dust and sand shari)ly against the faces of the buildings; 

 not with suflicient force, it may be, to perceptibly w^car 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 churchyards of Xew York City the efl'ects 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 orgcminms. — It is in such exposed situations, as 

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

 coatingof lichens or mosses, which by growing over its surface shield 

 it from the abrasive action. The full effect of growing organisms u[)on 

 the surface of stones is still, how-ever, a matter of dispute. By some 

 authorities H it is thought that they give rise to small amounts of organi<; 

 acids which exercise a coriosive 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 ben-eficial 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 U-ss 

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

 experiments of Winchell (Geo], of Minn., Vol. l, p. 11)7-201). 



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

 1'. IJlake. Proc. A. A. A. S., Providence meeting. 



t There is on exhibition in the National Museum a plate of glass fornu rly a. window 

 ill the light-house at Nausct 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 eomphjte over Hie entire .siuiace as tliongh done h}j 

 artificial means. 



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



II See Winchell, Geol, of MinR-, Yq\. ;, p. 188. 



