348 . -DYNAMICAL GEOLOGY. ~~ [Boox TI: 
and there largely evolved within the earth’s crust, especially in regions 
of extinct or dormant volcanoes. Subterranean water coming in the 
way of this gas dissolves it, and thereby obtains augmented solvent 
power. (3.) The capacity of water for dissolving mineral substances 
is augmented by increase of temperature (ante, p. 300). It is con- 
ceivable that cold springs containing a large percentage of mineral 
solutions may have acquired this impregnation at a great depth and 
at a higher temperature. As a rule, however, thermal water as it 
cools will deposit its dissolved minerals on the walls of the fissures 
up which it ascends. Hence no doubt the successive layers in 
mineral veins. (4.) Pressure likewise raises the solvent power of 
water (p. 300). (5.) Some of the solutions due to decompositions 
effected by the water, increase its ability to accomplish further de- 
compositions (p. 302). Thus the alkaline carbonates, which are. 
among the earliest products, enable it to dissolve silica and decompose 
silicates. These carbonates likewise promote the decomposition of 
some sulphates and chlorides. Calcium carbonate, which is found in 
the water of most springs, is the result of decomposition, and by its 
presence leads to the further disintegration of various minerals. 
“ Carbonic acid, bicarbonate of lime, and the alkaline carbonates bring 
about most of the decompositions and changes in the mineral 
kingdom. It is a matter of great importance to find that the same 
substances which give rise to so many decompositions in the mineral 
kingdom are the chief ingredients in the waters.”* 
‘The nature of the changes effected by the percolation of water 
through subterranean rocks will be best understood from an examina- 
tion of the composition of spring water. Springs may be conveniently 
though not very scientifically grouped into two classes. Ist: 
Common springs, such as are fit for ordinary domestic purposes, and 
2nd, mineral springs, in which the proportions of dissolved mineral 
matter are so much higher as to remove the water from the usual 
potable kinds. 
Common Springs possess a temperature not higher but fre- — 
quently lower than that of the localities at which they rise, and 
ordinarily contain, besides atmospheric air and its gases, calcic 
carbonate and sulphate, common salt, with chlorides of calcium and 
magnesium, and sometimes organic matter. The amount of dissolved | 
mineral contents in ordinary drinking water does not exceed °5, or at 
most 1:0 gramme per litre; the best waters contain even less. The 
amount of organic matter should not exceed from ‘005 to 01 gramme 
per litre in wholesome drinking water.’ Spring water containing a 
yery minute percentage of mineral matter, or in which this matter, 
even if in more considerable quantity, consists chiefly of alkaline 
salts, dissolves common soap readily, and is known in domestic 
economy as “soft” water. W here, on the other hand, the salts in 
solution are calcic or magnesic carbonates, sulphates, or chlorides, 
1 Bischof, Chem. Geol, i. p. 17. 
* Dr, B, H. Paul in Watts’ Dict, Chem. v. p. 1022. 

