BUILDING AND ORJ^AMENTAL STONES. 345 



Acid gases arc naturally without effect n])ou the silicious particles 

 of a saiulstoue, and can be i)rodiictive of injury only in dissolving out 

 the ferruginous and calcareous cements. This is actually accompliohed 

 in many cases, and much disintegration results as a consequence. In- 

 deed, Egleston* seems to regard the serious decay into which the stono 

 of Trinity Church, Kew York, has fallen, to be due chiefly to this cause, 

 supplemented by the action of frost after the cement had been removed 

 and the stone thus rendered porous. The relative solubility of the vari- 

 ous ferruginous cements has been already alluded to (ante p. o.'JD), Oxi- 

 dation is likely to piny a more noticeable part in sandstones than in most 

 other rocks, owing to their porous nature, which allows ready acci ss of 

 water and air. The effect of oxidizing pyrite in producing the mellow- 

 ing and other color changes in stones of this class is sufficiently dwelt 

 upon elsewhere, as is also the effect of heat, both natunil and artilicial. 



On account of their ])orosity and natural roughness of surface sand- 

 stones are of all stones most likely to afford foothold for the growth of 

 a1ga^, lichens, and mosses. While it is yet to be proven that these are 

 actually injurious, they are at least suggestive of an unhealthy damp, 

 ness. A stone once covered by these organisms will absorb more water 

 and give it up more slowly to evaporation than one whose surfaces are 

 not thus protected. 



Serpentines when free from bad veins are as a rule uon-absoiptive 

 and not affected by gaseous atmospheres, hence are durable if free IVom 

 bad joints. The Pennsylvania serpentines sometimes turn whitish on 

 exposure, but so far as observed do not disintegrate. 



Soapstone, although too soft and ]>ossibly too Nlii»i)ery for gene ral 

 building, is nevertheless one of the most durable stones, ])eing notolily 

 proof against atmospheric and chemical agencies, but when well sea- 

 soned tire-proof as well. 



(lyi)sum IS too soft and too soluble in ordinary teirestrial waters to 

 be of great value. 



I._OK THE SELECTION OF BUILDINC STONE. 



(1) GENERAL CONSIDERATIONS. 



From what has gone bei'ore it must be evident that there are many 

 more factors which go to determine the value of stone for striu'.tural 

 purposes than are ordimirily taken into consideration. It may there- 

 fore not be out of ])lace here to mention a few general ])rin<;ij)les to be. 

 observed in selecting stone for any purpose in which durability or 

 stability of color are matters of importance. It should be stated at the 



doubt, gtjt siitu lilted. Tliis means expeiidiiif;' more fuel to convert part of this wafer 

 into vapor. The difficulty is surmounted to a j;roat extent by l)ui]din<;- hollow walls, 

 the inn(>r wall beinj.; of brick. Wo<? unto the man who has not taken tins precaution." 

 (T. Mellard Reade, in Proc. Liverpool Geolo^rical Soc, p. 4 1.5 and 44(5, 188;5-H4.) 



*Caus(!andl'reventioii of Decay in JJuildin;^ Ston(i. Trans. Am. Soc. Civ. lOnjjs xv, 

 1886. 



