~ 
‘Parr IL, Secor. ii.§1.] ACTION OF RAIN. 331 
fertilized by the rain. Dr. Angus Smith cites the experience of M. 
J. J. Pierre, who found by analysis that in the neighbourhood of 
Caen, in France, a hectare of land receives annually from the 
atmosphere by means of rain—* 
Chloride of sodium . : : . : . 87:5 kilogrammes. 
75 
- potassium 8:2 
magnesium 2°5 
95 calcium . : : ‘ é ive 6 dS 
Sulphate of soda. : : ; tg OE 
iS potash . : ye OO 
m3 lime 6:2 
i magnesia D9 
Not only rain but also dew and hoar-frost abstract impurities 
from the atmosphere. The analyses performed by the Rivers 
Pollution Commission show that dew and hoar-frost condensing from 
the lower and more impure layers of the air are even more con- 
taminated than rain, as they contain on an average in Hngland 4°87 
parts of solid impurity in 100,000 parts, with -198 of ammonia.? 
It is manifest that rain reaches the surface by no means chemi- 
cally pure water, but having absorbed from the air various ingredients 
which enable it to accomplish a suite of chemical changes upon rocks 
and soils. So far as we know at present, the three ingredients which 
are chiefly effective in these operations are oxygen, carbonic acid, 
and organic matter. As soon as it touches the earth, however, rain 
begins to absorb additional impurities, notably increasing its propor- 
tion of carbonic acid and of organic matter, which it obtains from 
decomposing animal and vegetable matter. Among the organic 
products most efficaceous in promoting the corrosion of minerals and 
rocks are the so-called ulmic or humous substances that form 
soluble compounds with alkalies and alkaline earths, which are 
eventually converted into carbonates.* Hence as rain-water, already 
armed with gases absorbed from the atmosphere, proceeds to take 
up these organic acids from the soil, it is endowed with considerable 
chemical activity even at the very beginning of its geological career. 
Chemical and mineralogical changes due to rain- 
water.—In previous pages it was pointed out that all rocks and 
minerals are in varying degrees porous and permeable by water, that 
probably no known substance can under all conditions resist solution 
in water, and that the subsequent solvent power of water is greatly 
increased by the solutions which it effects and carries with it in its 
progress through rocks (pp. 298, 802). The chemical work done by 
rain may be conveniently considered under the four heads of 
Oxidation, Deoxidation, Solution, and Hydration. 
1, Oadation.—The prominence of oxygen in rain-water, and its 
1 Angus Smith, op. cit. p. 233. 2 Rivers Pollution Commission, 6th Rep. p. 32. 
> Senft, Z. Deutsch, Geol. Ges. xxiii. p. 665, xxvi. p. 954. This subject has recently 
been well treated in a paper by A. A. Julien “On the geological action of the humus 
acids” (Proc. Amer. Assoc, Xxviii. 1879, p. 311), to which further reference is made in 
later pages. 
