Irans. N. V. Ac. Set. ^4: Jan. 29, 



newer {e.g., the old City Hall); the few limestones yet brought into 

 use are beginning to lose their dressed surfaces and to be traversed 

 by cracks {e.g., the Lenox Library); and eveu the granytes, within a 

 half century, show both discoloration, pitting {e.g., the Custom House), 

 or exfoliation {e.g., the Tombs). To meet and properly cope with this 

 destructive action, requires, first, a clear recognition of the hostile ex- 

 ternal agencies concerned in the process. These belong to three 

 classes, chemical, physical, and organic. 



The chemical agencies discussed were the following : sulphurous 

 and sulphuric acids, discharged in vast quantities into the air of the city, 

 by the combustion of coal and gas, the decomposition of street refuse 

 and sewer-gas, etc.: carbonic, nitric, and hydrochloric acids ; carbolic, 

 Jiippuric, and many other organic acids, derived from smoke, street 

 •tlust, sewer vapors, etc.: oxygen and ozone, ammonia, and sea-salt. 



The mechanicil and physical agencies discussed were the following : 

 frost: extreme variations in temperature, amounting in our climate to 

 120*^ F. in a year, and even 70" in a single day : wind and rain, most 

 •efficient on fronts facing the north, northeast, and east : crystallization 

 by effl orescence : pressure of superincumbent masonry: friction: and 

 fire. 



Th^ organic agencies consist of ve^^etable growths, mostly confervse, 

 etc., within the city, and lichens and mosses, without, and of boring 

 molluscs, sponges, etc. 



The internal elements of durability in a stone depend, first, upon the 

 chemical composition of its constituent minerals and of their cement. 

 Tnis involves a consideration of their solubility in atmospheric waters, 

 €.g., the calcium-carbonate of a marble or limestone, the ferric oxide of 

 certain sandstones, etc. : their tendency to oxidation, hydra- 

 tion, and decomposition, e.g., of the sulphides (especially mar- 

 casite) in a roofing-slate or marble, the biotite and ferruginous 

 orthoclase in a granyte or sandstone, etc. : the enclosure of fluids and 

 moisture, e.g., as " quarry-sap," in chemical combination as hydrated 

 silicates (chlorite, kaolin, etc.) and iron-oxides, and as fluid-cavities 

 locked up in quartz, etc. 



The durability of a stone depends again upon its physical structure, 

 in regard to which the following points were discussed : the size, form, 

 and position of its constituent minerals ; e.g., an excess of mica-plates 

 in parallel-pasition rnay serve as an element of weakness: the porosity 

 of the rock, permitting the percolation of water through its interstices, 

 especially important in the case of the soft freestones and leading to 

 varieties of discoloration upon the light-colored stones, which were 

 -described in detail : the hardness and toughnes'=, particularly in relation 



