July 11, 1884.] 



* KNOWLEDGE ♦ 



35 



into liquid water, and thenceforward behaves precisely as 

 in the case of orilinary rain-water. 



In virtue of its absorbing properties, our drop of rain- 

 water, on reaching the surface of the earth through its 

 envelope of air and dust, has partaken of certain gases, such 

 as carbonic acid, oxygen, and a small proportion of nitrogen. 

 Carbonic acid gas preponderates, as it is more easily soluble 

 in water than the other gases ; it contains between thirty to 

 forty times more of that gas than the atmosphere. Of 

 solid impurities in England, it is estimated that there are 

 about 3-9.5 in 100000 parts.* In certain districts other 

 gases and substances are superadded, such as sulphurous 

 acid, ami sulphuretted hydrogen in the districts of active 

 volcanoes, and sulphuric and nitric acids in the vicinity of 

 manufacturing towns. It is needless to observe that water 

 thus charged would tend very largely to modify the surface 

 of the earth generally, and buildings, Ac, more markedly, 

 by a conversion, e r/ , of the lime of mortars, marble monu- 

 ments, ifcc, into sulphate of lime through the action of the 

 sulphuric acid. Examples of such action have been care- 

 fully recorded by Angus Smith, + and more recently by 

 Geikie. J Sulphates of alkalies and alkaline earths, and 

 chlorides, especially common salt in the neighbourhood of 

 coasts, swell tlie list of impurities which crowd into a drop 

 of water before it commences its work upon the crust of 

 the earth. Other inorganic particles which have been 

 detected vai-y in character with the surrounding circum- 

 stances ; amongst them may be mentioned specks of tine 

 terrestrial dust, and dust of cosmic origin, containing 

 minute spherules of iron.'! Some of the organic impurities 

 of rain-water which have been derived from the air were 

 discoursed u|ion at length in our last conmiinication. 



Thus fortitied, rain-water is peculiarly adapted to com- 

 mence its work of modification upon the crust of the earth. 

 Its carbonic acid is augmented in quantity, and it thus 

 becomes a powerful solvent, particularly upon substances 

 composed of or containing lime salts. Xow, since vast 

 tracts of country are formed of limestones, marbles, chalks, 

 and other calcareous rocks, its action must indeed be enor- 

 mous. Large quantities of lime are dissolved and carried 

 away by the water which percolates through the earth 

 to its Tinderground reservoirs. In some instances these 

 calcareous waters drop through caverns, and so highly are 

 they charged with lime that evaporation of the water, as it 

 leaves the roof of the cave and reaches the floor, is sufficient 

 to deposit the lime in concentric rings, which afterwards 

 take on a radiated structure, and give rise to more or less 

 complete pillars, some dependent, and some in the form of 

 upright bosses on the floor, — stalactites and stalagmites. 

 Deposition through evaporation also goes on in rock fissures 

 and cavities ; veins and druses full of crystals, sometimes 

 of rare minerals in exquisitely developed and perfect forms, 

 are thus evolved. A quantity of the water is often held in 

 these crystals as water of crystallisation, and subsequent 

 action, such as that of heat or pressure, may entirely 

 change the character of these deposits ; instances may be 

 found in the mineral calcite and its allied forms. 



The influence of the carbonic acid of rain-water upon 

 rock structure h sometimes most complicated, and a vast 

 field of inquiry lies open before the experimental petro- 

 logist in tliis direction. As an e.\ample, we may point to 

 the large group of metamorphic rocks of which the serpen- 

 tines are the most noteworthy ; here§ the carbonated 



* Sixth Report of the Rivers Pollution Commission, p. 29. 

 t " Air and Rain," 1872, p. 444. 

 J " Proc. Rov. Soc. Edin., 1879-SO," p. 518. 



II Jung, " Bull. Soc. Vandoise Sei. Xat.," xiv., p. 493 : Ton Lasaulx 

 in "Techermak's Mineral. Mittheil.," 1880, p. 517. 

 § Dana, " System of Mineralogy," 5th Ed., p. 258. 



waters appear to remove the oxidised iron and magnesia 

 of the mineral olivine (in serpentines derived from olivine- 

 rock.s), as precursory to its conversion into the beautiful 

 green material which is so much admired in architectural 

 decorations and ornaments. Even the silicates, as of lime, 

 soda, and pota^li, crumble away in time before its action ; 

 and thus are the felspars of granite and diorite broken 

 down into sands and clays ; their alkalies are removed as 

 soluble carlionates, and the silica is generally liberated in 

 the form of a hydrated bi.silicate of alumina called kaolin, 

 a substance not unknown to Messrs. Doulton it Co., of 

 Lambeth, whose exhibits we recently reviewed. 



The oxygen taken up by rain-water is also added to 

 during its percolation through the soil. Its action upon, 

 calcareous rock-masses is not of very great importance, 

 unless they happen to contain a large proportion of iron ;. 

 but in regions where the rocks are chiefly of igneous 

 origin, as in Devon, Cornwall, the lake district, Cymric 

 mountains, and Scottish Highlands, minerals such as horn- 

 blende, augite, olivine, apatite, ic, occur largely, and the 

 oxygen plays an important ])art in the conversion of certain 

 of the mineral components into ferrous and manganous 

 oxides. The rocks thus weather or crumble away to form 

 soils through whicli future waters penetrate, and from 

 which they take their character. The formation of soil 

 from volcanic rocks which are rich in a|)atite or phos- 

 phate of lime, through this chemical disintegrative action 

 of water, is of the first importance to the agricultural' 

 economist. 



Our remarks upon the organic impurities in water wer& 

 confined to such of them as are chiefly derived from the air 

 in the germ condition. On reaching the earth, however, rain- 

 water becomes the vehicle for enormous quantities of other 

 organic matters which exercise profound chemical changes ^ 

 they act upon minerals to a degree which would scarcely be 

 credible at tirst sight. The so-called humus or ulmic acids, 

 which they create, combine with mineral bases to forni 

 soluble substances which are afterwards changed into car- 

 bonates. A detailed account of these reactions may be 

 gleaned from the researches of Senft* and Julien.f 



What is more ap]iarent, however, is the reducing action 

 of water through the agency of contained organic matter. 

 Any one who has visited districts abounding in red sand- 

 stones, as along the northern and eastern coasts of Scot- 

 land, would be struck by the numerous whitish and 

 greenish-white spots and streaks which mottle the rock 

 every here and there. Tiie cause of these discolorations 

 has been explained thus : — The organic matter in the water 

 abstracts oxygen from the ferric oxide, which imparts the 

 red colour to the stone, and reduces it to the condition of 

 a ferrous oxide, which is thereafter removed by becoming » 

 soluble carbonate or an organic salt. Organic matter 

 also acts as a deoxidiser by the conversion of sulphates 

 into sulphides, and is thus capable, for instance, of com- 

 pletely metamorphosing districts composed of sulphate of 

 lime, &c. Whilst speaking of gypsum, or sulphate of 

 lime, we are reminded that a certain proportion dissolved 

 in water gives the latter undesirable characters which are 

 often difficult to eradicate ; we shall have occasion to 

 discuss this subject when we come to treat of the practical 

 details of water supplies. Gypsum contains a large 

 quantity of water in its composition ; when deprived of 

 this water it is called anhydrite, and in becoming thus 

 changed it loses in bulk : this is a feature of considerable 

 interest to the dynamical geologist, who fjnds in it an ex- 

 planation for unaccountable upheavals in districts where 



* " Zeits. Dent. Geol. Gea.," xxiii., p. 665 ; xxvi., p. 945. 

 t " On the Geological Action of the Humus Acids." Proc. Amer, 

 Assoc, xxviii., 1879, p. 311. 



