336 



EEPOET ON NATIONAL MUSEUM, 1886. 



tliey grow iiud its enviromnent. Mure observations are necessary be- 

 fore aitytbing detiiiite can be said.* 



(2) CHEMICAL AGENCIES. 



Composition of the atmosphere. — Tbe atmosi)here in its normal state 

 consists of a mecbanical admixture of nitrogen and oxygen in about tbe 

 proi)ortions of four volumes of tbe former to one of tbe latter, togetber 

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

 Jn tbe vicinity of large manufacturing cities, bowever, it carries iu addi- 

 tion to increased proportions of carbonic acid,t appreciable quantities 

 of sulpburoug, sulpburic, nitric, and bydrocblori(; acids. Tbese, wbeu 

 brougbt by raius into contact witb tbe Avails of buildings, are capable, 

 tbrongboui many years of time, of producing maibed eifects, especially 

 wlien aided by tbe extreme diurnal ranges of temperature common in 

 tbe eastern and nortbern United States.| 



* The vegetation of microscopic liclieus tates place upon the surface of the stone, 

 when, from any cause, that surface becomes roughened so as to atforil a hxlgmeut foi' 

 the see<Ls or sjtorcs of these phiuts. These growing, still further hasten the disinte- 

 gration of the stone, and accumulating about them the fine dust floated by the at. 

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

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

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

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

 jiosed 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 

 aud with a more or less elevated situation. 



It should not bo 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 i)lants. The lichen-covered 

 jOcks 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 aud .%.) 



t Twenty-one tests of the air iu various parts of Bo.ston during the spring of lyJU 

 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 ])arts of the acid iu 1,000,000 (Second An- 

 nual Keport 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 

 Siuith (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 iu 1,000,000. 



I Dr. Smith {op. cii.) found the proportious of these acids in London, Liverpool, ■ 

 aud Manchester to be as follows : 



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

 jallon. It should be noted, however, that these acids were not considered as existing 



