336 



REPORT ON NATIONAL MUSEUM, 1886. 



they grow and its environment. More observations are necessary be- 

 fore anything definite can be said.* 



(2) CHEMICAL AGENCIES. 



Composition of the atmosphere. — The atmosphere in its normal state 

 consists of a mechanical admixture of nitrogen and oxygen in about the 

 proportions of four volumes of the former to one of the latter, together 

 with minute quantities of carbonic acid, ammonia, and vapor of water. 

 In the vicinity of large manufacturing cities, however, it carries in addi- 

 tion to increased proportions of carbonic acid,f appreciable quantities 

 of sulphurous, sulphuric, nitric, and hydrochloric acids. These, when 

 brought by rains into contact with the walls of buildings, are capable, 

 throughout many years of time, of producing marked eifects, especially 

 when aided by the extreme diurnal ranges of temperature common in 

 the eastern and northern United States. | 



* The vegetation of microscopie lichens takes place upon the surface of the stone, 

 when, from any cause, that surface becomes toughened so as to afford a lodgment for 

 the seeds or spores of these plants. These glowing, still further hasten the disinte- 

 gration of tlie stone, and accumulating about them the line dust floated by the at- 

 mosphere becomes points for the absorption of more water, which, on freezing, still 

 further roughens the surface, and the patch of lichen gradually extends. These 

 lichens often gain attachment upon the surface of a tinely dressed stone, from some 

 little inequality of texture, or from softer material that more readily becomes decom- 

 posed or more readily accommodates the growth of the plant. Such, stones in time 

 become partially, or entirely covered by lichens, and present an unsightly aspect. 

 The amount and degree of this growth varies with position in reference to the sun 

 and with a more or less elevated situation. 



It should not be forgotten, however, that any stone giving root to lichens is not 

 one of those which most easily disintegrates, for in these the destruction goes on so 

 rapidly that the surface does not allow the growth of such plants. Thelichen-coveied 

 r ock8 in nature are usually those of great strength and durability. None of the softer 

 or rapidly decaying rocks produce this vegetation. (Rep. on Building Stones by 

 James Hall, 1868, pp. 54 and 55.) 



t Twenty-one tests of the air in various parts of Boston during the spring of 1870 

 yielded Mr. Pearson 385 parts of carbonic acid in 1,000,000. Eleven tests of the win- 

 ter air of Cambridge yielded Mr. Hill 337 parts of the acid in L, 000,000 (Second An- 

 nual Eeport Massachusetts State Board of Health, 1871, p. 52). Dr. Kidder found the 

 outdoor air of Washington to contain from 387 to 448 parts in 1,000,000. Mr. Angus 

 Smith (Air and Rain, p. 52), after an elaborate series of experiments, reports the air 

 of Manchester (England) to contain on an average 442 parts of the acid in 1,000,000. 



I Dr. Smith {op. cit.) found the proportions of these acids in London, Liverpool, 

 and Manchester to bo as follows : 





Sulphuric. 



Hydrochloric. 



Nitric. 



Localities. 



Grains per 

 gallon. 



Parts per 

 million. 



Grains per 

 gallon. 



Parts per 

 million. 



Grains per 

 gailou. 



Parts per 

 million. 





1. 4345 

 2.7714 



2. 0163 



20.49 

 39. 59 

 41. 06 



.0872 

 .7110 

 .4055 



1.250 

 10.16 

 5.79 





.840 



Liverpool 





.582 





.886 









He also found the total acids for Manchester to average for 1870 3.7648 grains per 

 gallon. It should bo noted, however, that these acids were not considered as existing 



