PRINCIPLES OF AGRICULTURAL CHEMISTRY 



In exceptional cases it has been known to contain so much dust 

 as to assume a red or black color. It usually contains a small 

 amount of chloride of sodium, and sulphates. The following is 

 a summary of a large number of analyses of rain water made by 

 Angus Smith. 1 



ANALYSES OF RAIN WATER PARTS PER MILLION. 



We find from the table that the coast rain water contains more 

 salt (sodium chloride) than the rain of inland places. This is 

 due to the salt spray from the sea, which is broken up into fine 

 particles, and carried by air currents for long distances. The 

 effect of the sea upon rain water is often noticeable for a hun- 

 dred miles inland. 



Comparing rain water of the city and of the country, we find 

 that the former is marked by the presence of considerably more 

 sulphates and ammonia, and that it also contains free acid. The 

 increased quantity of sulphates and the sulphuric acid in the 

 rain water of cities can be traced to the combustion of coal con- 

 taining sulphur. 



Arid and Humid Climates. In a humid climate, the rainfall is 

 sufficient, or more than sufficient, for the production of culti- 

 vated crops. In an arid climate the rainfall is insufficient in 

 quantity, and crops can be grown only through irrigation, or 

 by means of special methods of culture. The character of the 

 rainfall also influences the relation of the climate towards crops. 

 A comparatively small amount of rain distributed through the 

 growing season may give a locality the characteristics of a humid 

 region, while a heavier rainfall so distributed that very wet 

 periods are followed by long intervals of little or no precipita- 

 1 Jour. Chem. Soc., 1872, p. 33. 



