ON THE DURABILITY OF BUILDING STONE 373 



VI 



ON THE DURABILITY OF BUILDING STONE 

 AND THE CAUSES OF DECAY 



I. Physical Structure — State of Aggregation of 



THE Particles 



The physical structure and the chemical composition of 

 stone are so related, and the durability is so dependent 

 upon them, that it is difficult to consider them separately. 

 The rapid disintegration of a coarse-granular and loosely- 

 aggregated stone is hastened if the component grains be 

 readily soluble in water containing acid gases. Conversely, 

 the resistance to weathering agents, in the case of a siliceous 

 stone, is vastly increased when its compact mass is so dense 

 that there are no interstitial spaces into which water can 

 penetrate rapidly. The points of attack are lessened by 

 the closer aggregation of the crystals or grains. So inti. 

 mate is this relation of structure and composition that the 

 defects of the one class are not offset by the advantages of 

 the other, and these defects impair seriously the value of a 

 stone — the ideal structure is counterbalanced by wanting 

 strength of chemical constitution. 



The physical structure is of importance in the two great 

 classes of stone, the crystalline and the granular or sedi- 

 mentary ; although in the latter class the results of defec- 

 tive or wanting constitution are, perhaps, sooner apparent 

 than they are in the former — crystallines. 



I. Sedimentary or granular rocks or stone. — The size and 

 arrangement of the grains are here considered. It is evi- 

 dent that the size of the grains determines to some extent 

 the interstitial spaces and the porosity of the mass, inas- 

 much as the coarser-granular varieties leave larger spaces, 



