II. 



THE ATMOSPHERE 



25 



Nitric Acid, or some compound of nitrogen and oxygen, is also 

 found in air, in which it probably exists in the form of nitrate or 

 nitrite of ammonia, and, according to Muntz and Aubin, 1 these in the 

 state of finely-divided solid. The authors quoted state that thunder- 

 storms (and the accompanying formation of nitrates) are confined 

 to the lower portion of the atmosphere, below 3000 metres, and that 

 above that height no nitrates are found in rain or snow. 



The amount of nitric acid in air is so small that it can only be de- 

 tected, as a rule, by examination of rain-water, in which it becomes 

 more concentrated. 



Rain-water, too, brings down ammonia and suspended matter, so 

 that analyses of rain-water are valuable as indications of the com- 

 position of the atmosphere. Many such analyses have been published. 

 Thus Angus Smith' 2 in 1872 gave the following result of a large 

 number of analyses : 



ANALYSES OF RAIN-WATER, 

 PARTS PER MILLION. 



In these analyses it is to be noted that the chlorine is mainly due to 

 the dried-up particles of sea-water cast into the air as spray. The amount 

 is always greatest near the sea and smallest far inland ; it is greatly 

 influenced by the direction and force of the wind. Sulphuric acid is 

 also partly attributable to the same cause, but if its ratio to the chlorine 

 be more than 12 : 100 (the ratio in sea-water) combustion of coal or 

 the decay of animal or vegetable matter is probably to be assigned as 

 one of its sources. 



Another extensive series of analyses of rain-water collected at 



1 Compt. Rend., 95, 919. 



2 Jour. Chem. Soc., 1872, 33. 



