136 THE AIR 



(e) Compounds of Nitrogen. — These compounds are of 

 importance as far as they go, but the amount present is very 

 small. They are principally oxides of nitrogen and am- 

 monia, occurring usually, perhaps, as nitrous and nitric 

 acids and ammonium nitrite, nitrate, carbonate, or sulphate. 

 The oxides of nitrogen are formed from oxygen and nitrogen 

 by lightning discharges. The intense heat in the immediate 

 vicinity of the electric spark causes a very small part of 

 these gases to unite. Ammonia is formed by the decomposi- 

 tion of organic matter, although free nitrogen is formed 

 under some conditions (Section 129). These compounds of 

 nitrogen can be used very well by plants after some changes 

 in the soil, but the amount is hardly worth considering 

 ordinarily. On the average there are about three pounds of 

 nitrogen brought to the surface of an acre in a year by rain 

 and snow. Occasionally this may amount to ten pounds, 

 but very rarely. 



(/) Bacteria. — The solid particles of the air, besides the 

 nitrate, carbonate, and sulphate of ammonium mentioned 

 above, consist of bacteria and dust. Bacteria exist in count- 

 less numbers in the air, invisible, but nevertheless of great 

 importance, sometimes beneficial, sometimes harmful. The 

 bacteria which aid in decomposing organic matter in the 

 soil and in making nitrogen available are all carried in the 

 air and help the farmer very materially. Moreover, the 

 germs of many diseases are carried through the air and work 

 considerable harm. One proof of the presence of bacteria in 

 the air is to be found in the fact that if perfectly sterile milk 

 is exposed to the air for any length of time it will sour, due 

 to the lactic acid bacteria being carried to it by the air. 



(g) Dust. — Fine particles of dust are everywhere present 

 in the air, and consist of minute particles of organic matter, 

 bits of cotton, pieces of hair, and fragments of minerals. 

 There may also be pollen from flowers, and spores of fungi. 

 These particles of dust form nuclei for the precipitation of 

 water vapor and hence cause the formation of fog and clouds. 

 Dustless air would contain no fog. 



(h) Sulphur Dioxide is of very serious consequence in 

 some places. In the western part of the United States where 



