710 STUDIES FOR STUDENTS 



general mean of 278 parts in a million for stations in Florida, 

 Mexico, Haiti, Chile and Patagonia, and 296 parts in the 

 north of France. Fischer as quoted by Branner ^ has shown 

 that in rain and snow water the amount of carbonic acid varies 

 between 0.22 per cent, and 0.45 per cent, by volume of water. 

 Assuming that the mean of these figures fairly represent the 

 general average, it is easy, knowing the rainfall of any region to 

 calculate the amount of gas thus annually brought to the surface. 

 Professor Branner has thus calculated that from 3.21 to 11.80 

 millimeters of carbonic acid gas (COg) are annually brought to 

 the surface in certain parts of Brazil. The same method of cal- 

 culation applied to the various parts of the United States, would 

 give us for the Atlantic coast states 3™™. 75 ; for the upper Missis- 

 sippi Valley 2"^"". 5; for the Lower Mississippi Valley 4""". 5, and 

 for the northern Pacific states 6™"". 25. As it is mainly when 

 this carbonic acid is thus brought to the surface by rain and snows 

 that its effects become of direct significance in our present work, 

 we may drop the matter here, to be taken up again when con- 

 sidering the chemical action of water. 



Oxyge7i. — Under ordinary conditions oxygen is the most 

 active principle in the atmosphere, and it is to this agent that 

 we owe the process of oxidation whereby silicates and other 

 minerals containing iron in the protoxide state undergo decom- 

 position. Even here, however, oxidation is almost inactive 

 unless aided by moisture and a further discussion of the subject 

 may well be deferred to be taken up again when discussing the 

 action of water. 



Heat and cold. — The ordinarily feeble action of the air is 

 greatly augmented through natural temperature variations. That 

 heat expands and cold contracts is a fact too well known to 

 need elaboration here. That however the constant expansion 

 and contraction due to dmrnal temperature variations may be 

 productive of weakness and ultimate disintegration in so inert a 

 body as stone, seems not so generally understood, or is at least 

 less well appreciated, and hence a little space is devoted to the 



' Bull. Geo!. Soc. of Am., Vol. VII, 1896, p. 305. 



