CAUSES OF DECAY 383 



portance. Exposure to the direct rays of the sun produces 

 a greater variation of temperature than that on north or 

 shaded sides of buildings. TJie range of temperature in our 

 northern states is comparatively great and subject to sharp 

 and frequent fluctuations. Stone has not a high conductive 

 power, and, generally, little elasticity. In the sedimentary 

 rocks the grains are, in most cases, uniform in the nature of 

 material, but in the crystalline rocks there is more generally 

 an aggregation of minerals of diverse species, each of which 

 has its rate of expansion. The coefficient of expansion in 

 the case of quartz, for example, is much greater than it is in 

 orthoclase feldspar and hornblende ; calcite is much less 

 than that of dolomite. This unequal rate of expansion, 

 where the range of temperature Is great, tends to the pro- 

 duction of slight fractures and interstices, Into which 

 moisture and air can penetrate, and the strength of the 

 crystalline mass is, no doubt, impaired in time by many al- 

 ternations of heat and cold, and the consequent expansion 

 and contraction to which it Is subjected. As is well known, 

 some of the granites fly to pieces more quickly when sub- 

 jected to a high temperature than the sandstones. The 

 cause may be in this differential rate of expansion. The 

 failure of granite, in the great Boston fire, Is a notable ex- 

 ample of the unequal tension and the consequent destruc- 

 tion, in case of Intense heat."^* The tests reported in the 

 table show the disastrous effects of heat on granites as com- 

 pared with sandstones of even grain. 



Held together by their cohesive power, the Individual 

 blocks of stone expand as units, and stone structures of large 

 size, no doubt, also deviate from a normal, according to the 

 degree of heat. Thus the rate of expansion for granite Is 

 .000004825 Inch per foot, per degree of F. ; that of marble, 



*The great fire of Boston, like that of Chicago, was an extraordinary test, and the 

 heat was more intense than that of a lesser fire, and no stone withstood it success- 

 fully. Neither sandstone nor even common building brick can stand up in a blast 

 furnace. 



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