Kin ah an — On Irish Arenaceous Rocks. 7 



volves a consideration of their solubility in atmospheric waters ; 

 e.g. the calcium (carbonates of a marble or limestone), the ferric 

 oxide of certain sandstones, &c. ; their tendency to oxidation, 

 hydration, and decomposition, e. g. of the sulphides (especially 

 marcasite), in roofing-slates, limestones, &c. ; the biotite and ferru- 

 ginous orthoclase in a granite or sandstone, &c. ; the enclosure 

 of fluid and moisture, e. g. as ' quarry sap ' in chemical combina- 

 tion — as hydrated silicates (chlorite, kaolin, &c.) and iron oxides, 

 and as fluid-cavities locked up in quartz," &c. The durability of a 

 stone depends also upon its physical structure in regard to the 

 following, viz. : the size, form, and disposition of its constituents. 

 For instance, an excess of mica plates in parallel position may be 

 an element of weakness ; the porosity of the rock permitting the 

 percolation of water through its interstices ; the hardness and 

 toughness; the crystalline structure ; the tension of the grains (which 

 appears especially to explain the disruption of many crystalline 

 marbles) ; the contiguity of the grains, and the proportion of cement 

 in their interstices, and the homogeneity of the rock. " Again, the 

 durability of a rock may depend on the character of its surface, 

 whether polished, smoothly dressed, or rough hewn ; since upon 

 these circumstances may depend the rapidity with which atmo- 

 spheric waters are shed, or with which the disposition of soot, 

 street-dust, &c, may be promoted; also on the inclination and 

 position of the surface, as affecting the retention of rain-water and 

 moisture, exposure to northerly gales and to burning suns," &c. 



The agencies concerned in the process of destruction belong to 

 three classes, viz. chemical, physical, and organic. The chemical 

 agents are sulphurous and sulphuric acids, discharged in vast 

 quantities into the air of a city from the combustion of coal and 

 gas, sewer gas, the decomposition of street refuse, &c. ; carbonic, 

 nitric, and hydrochloric acids ; carbolic, hippuric, and many other 

 acids, derived from smoke, street-dust, sewer exhalations, &c. ; 

 oxygen, ozone, ammonia, and sea-salt. 



The more prominent mechanical and physical agencies are : 

 extreme variations in temperature, wind and rain (to which some 

 faces of a building are more exposed than others), crystallization 

 by efflorescence, pressure of superincumbent masonry, friction, and 

 fire. 



The natural method of ascertaining the durability of stone, 



