+ oe =) 
wi 
330  . . DYNAMICAL GEOLOGY. — [Boox TIL 
there. (2.) It acts mechanically, by washing away loose materials, 
and thus powerfully affecting the contours of the land. 
1. Chemical Action.—This depends mainly upon the nature and 
proportion of the substances abstracted by rain from the air in its 
descent to the earth. Rain absorbs a little air, which always con- 
tains carbonic acid as well as other ingredients, in addition to its 
nitrogen and oxygen (p. 31). Rain thus washes the air and takes 
impurities out of it, by means of which it is enabled to work many 
chemical changes that it could not accomplish were it to reach the 
ground as pure water. 
Composition of Rain-water.—Numerous analyses of rain- 
water show that it contains in solution about 25 cubic centimetres of 
eases per litre.' An average proportional percentage is by measure 
—nitrogen, 64:47; oxygen, 33°76; carbonic acid, 1°77. Carbonic 
acid being more soluble than the other gases is contained in rain- 
water in proportions between 30 and 40 times greater than in 
the atmosphere. Oxygen too is more soluble than nitrogen. This 
difference acquires a considerable importance in the chemical 
operations of rain. Other substances are present in smaller quan- 
tities. In England there is an average of 3°95 parts of solid im- 
purity in 100,000 parts of rain. Nitric acid sometimes occurs in 
marked proportions: at Bale it was found to reach a maximum of 
13°6 parts im a million, with 20:1 parts of nitrate of ammonia. 
Sulphuric acid lkewise occurs especially in the rain of towns and 
manufacturing districts.? Sulphates of the alkalies and alkaline 
earths have been detected in rain. But the most abundant salt is 
chloride of sodium, which appears in marked proportions on coasts, 
as well as in the rain of towns and industrial districts. Rain taken 
at the Land’s End in Cornwall during a strong south-west wind was 
found to contain 2°180 of chlorine, or 3:591 parts of common salt in 
every 10,000 of rain. The mean proportion of chlorine over England 
is about 0022 in every 10,000 parts of rain; at Ootacamund 0-003 
to 0:004.4 ; 
In washing the air rain carries down also inorganic particles or 
motes floating there; likewise organic dust and living germs.® As 
the result of this process the soil comes to be not merely watered but 
* Baumert, Ann. Chem. Pharm. \xxxviii. p.17. The proportion of carbonic acid found 
by Peligot was 2:4. Sce also Bunsen, op. cit. xciii. p. 20. Roth, Chem. Geol. i. p, 44. 
Dr. Angus Smith’s Air and Rain, 1872, p. 225. 
* Rivers Pollution Commission, 6th Rep. p. 29. 
* The occurrence of sulphuric and nitric acids in the air, especially noticeable in 
large towns, leads to considerable corrosion of metallic surfaces, as well as of stones and 
lime. ‘The mortar of walls may often be observed to be slowly swelling out and dropping 
off, owing to the conversion of the lime into sulphate. Great injury is likewise done from 
a similar cause to marble monuments in exposed graveyards. See Dr. Angus Smith, op. 
cit, p. 444. Geikie, Proc. Roy. Soc. Ldin. 1879-80, p. 518. 
* Dr, Angus Smith, op. cit. Livers Pollution Commission, 6th Rep. 1874, p. 425. 
* Among the inorganic contents of rain and snow fine dust and spherules of iron, 
probably in part of cosmic origin, have been specially noted. See Jung, Bull. Soc. 
Vaudoise Sci. Nat. xiv. p. 493, authorities cited ante p. 64; Von Lasaulx, as cited on 
p. 826, The organic matter is revealed by the putrid smell which long-kept rain-water 
gives out, 

