INTRODUCTORY 15 



constituents of the atmosphere to eliminate the water, and 

 except when otherwise stated, remarks on this subject 

 generally refer to dry air. 



Carton Dioxide. This substance, which is present to 

 the extent of four parts in ten thousand of dry air, i.e., 

 0'04 per cent., is not, like oxygen and nitrogen, an element. 

 It is a compound body, and, as the name implies, it con- 

 tains the elements carbon and oxygen chemically united. 

 It is produced by the burning of fires, the respiration of 

 animals and other processes of oxidation of carbon and 

 carbon compounds. It is frequently, though not with strict 

 accuracy, called carbonic acid because it combines with 

 lime and other bodies of that class forming carbonate of 

 lime, etc. The carbon and oxygen are very firmly united 

 and are not easily separated. All green plants, however, 

 can decompose the compound. When exposed to bright 

 sunlight they take up the carbon dioxide from the air and 

 disunite the elements, giving off oxygen and retaining 

 the carbon to build up their tissues. 



Impurities. The impurities in air may be estimated both 

 qualitatively and quantitatively by examination of the rain 

 water in which they are brought down either mechanically 

 or in solution. Besides the nitric acid and ammonia pre- 

 viously mentioned, they consist mainly of the chlorides 

 and sulphates of sodium, magnesium and calcium all of 

 which are more or less soluble some oxide of iron, alu- 

 mina and silica, which are insoluble. In addition to these 

 inorganic particles there are also numerous micro-organ- 

 isms, such as those which cause the souring of milk, 

 various kinds of decay and putrefaction, and certain 

 diseases such as tuberculosis. 



The following table shows the quantity of nitrogen in 

 the form of ammonia and of nitric acid in the rain water 

 at various places. The results are given both as parts per 

 million and as Ibs. per acre. 



