PRINCIPLES OF ROCK WEA THERING 709 



growinu; j_)l;ints aiul it is j^robable that a very large proportion 

 of that which finds its way there, is quickly taken up again by 

 these organisms, and but little is left to promote decay. It is 

 possible that other salts of ammonium than the nitrate may be 

 locally efficacious. Thus M. Beyer as quoted by Van Den 

 Broeck' has shown that the feldspars decompose very rapidly 

 under the influence of water containing ammonium sulphate or 

 e\'en sodium chloride, either of which substance may be found 

 in vegetable soil. 



Carbo)iic acid. — The amount of carbonic acid in the air under 

 natural conditions is not a widely variable quantity excepting near 

 volcanoes and the immediate vicinity of gaseous springs. This 

 has been pretty thoroughly demonstrated by Muntz and Aubin.^ 

 In the vicinity of large cities and manufactories consuming great 

 quantities of coal the amount is naturally increased. Although 

 carbonic acid is the most abundant gas given off by decomposing 

 vegetable matter, it has apparently been definitely ascertained that 

 the amount of this gas in the atmospheres of regions of abun- 

 dant vegetation is no greater than elsewhere. This has been 

 accounted for on the assumption that the gas as fast as liberated 

 is taken up by growing organisms or carried by rains into the soil. 

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

 spring of 1870, showed the presence of 385 parts of carbonic 

 acid in 1,000,000. 



Eleven tests of the winter air in Cambridge yielded 337 parts 

 in 1,000,000.3 Dr. J. H. Kidder found the out-door air of 

 Washington to contain 387 tc 448 parts in 1,000,000, while Dr. 

 Agnus Smith after an elaborate series of experiments, reported 

 the atmosphere of Manchester (England) as containing 442 

 parts in 1,000,000.'' These tests are all of atmospheres in the 

 vicinitv of cities. Muntz and Aubin, quoted above, found a 



'Mem. Sur. Les Phenomes D'Alteration Des Depots Superficial, p. 16. 

 = Muntz and Aubin. Comptes Rendus. 93. 1881, p. 797. Also 96. 1883. 

 PP- 179^-97- 



3 Second Annual Report Massachusetts State Board of Health. 1871. 

 ■•Air and Rain, p. 52. 



