4:8 SCIENCE OF COMMON THINGS. 



Durability of building materials. Effect of the atmosphere on rocks. 



The middle part of the entablature. 



S86 What is the cornice f 



The upper or projecting part of the entablature^ 

 (For illustration of these different terms, see fig. 13.) 



28*7 In selecting a stone for architectural purposes, how may we be able 

 to form an opinion respecting its durability and permanence? 



By visiting the locality from whence it was obtained, 

 we may judge from the surfaces which have been long 

 exposed to me weather if the rock is liable to yield to 

 atmospheric influences, and the conditions under which 

 it does so. 



" For example, if the rock be a granite, and it be very uneven and 

 rough, it may be inferred that it is not very durable; that the feldspar, 

 which forms one of its component parts, is more readily decomposed by 

 the action of moisture and frost than the quartz, which is another ingre- 

 dient ; and therefore that it is very unsuitable for building purposes. 

 Moreover, if it possesses an iron-brown or rusty appearance, it may be 

 set down as highly perishable, owing to the attraction which this metal 

 has for oxygen, causing the rock to increase hi bulk, and so disintegrate." 



388 Why are the sandstones, termed freestones, ill adapted for the 

 external portions of exposed buildings f 



Because they readily absorb moisture ; and in coun- 

 tries where frosts occur, the freezing of the water in 

 the wet surface continually peels off the external por- 

 tions, and thus, in time, all ornamental work upon the 

 stone will be defaced or destroyed. 



389 Why do some species of rock become harder when taken from the 

 quarry and exposed to the atmosphere f 



This quality, in some species of stone, arises from 

 the fact that the water contained in it, when forming 

 part of the natural rock, evaporates, and the stone, 

 becoming dryer, becomes harder. 



390 Why do some stones, altlwugh hard when first quarried, become 

 friable, and fall to pieces, when exposed to the atmosphere '( 



Because they contain clay or alumina in such a state 

 as to readily absorb moisture from the atmosphere ; and 

 through the agency of the moisture the particles lose 

 their cohesion and tall apart. 



