VIII.] COMPOSITION OF NATURAL WATERS. 117 



temperature and pressure, 100 volumes of water dissolve 

 1-48 volumes of nitrogen, 2'99 volumes of oxygen, ioo'2 of 

 carbonic acid, and 78270 of ammonia. All the constituents 

 of the atmosphere will therefore be found in a state of 

 solution in rain-water ; while other bodies, such as nitric 

 acid, also derived from the atmosphere, are not unfrequently 

 present. In fact, whatever soluble constituents exist in the 

 air will be absorbed by the rain. Hence, in the neighbour- 

 hood of towns, where the atmosphere is impure, the rain- 

 water will wash out more or less of its impurities, and, 

 consequently, the rain collected in populous districts is less 

 pure than that collected in an open country. Moreover, the 

 rain which falls at the beginning of a shower is more 

 contaminated than the later rain ; and rain, which falls 

 after a long drought, is more impure than that which falls 

 towards the close of a rainy season. But, even after a long 

 continuance of wet weather, the rain will still contain 

 atmospheric gases to the extent of about 2| cubic inches 

 to 100 cubic inches of water. 



When the rain reaches the surface of the fearth, it im- 

 mediately commences to attack the rocks on which it happens 

 to fall. Whether it dissolves much or little will depend 

 upon whether the earth contains more or less soluble 

 matter. But, whatever the character of the ground, some- 

 thing will certainly be always dissolved. Every runnel, 

 every brook, every rivulet, thus washes out some of the 

 soluble constituents of the rock over which it flows, and 

 carries them onwards to the river. The river consequently 

 becomes the common receptacle for all the soluble matter 

 delivered by its tributary streams. As it flows along, it 

 grows richer in these soluble constituents, deriving them 

 partly from the wear of its own bed and partly from that of 

 its banks. It is not, however, by merely running over the 



