WATER AS A CHEMICAL AGENT. 705 



B. Combinations of the Hydrogen of Water, at the Ordinary Tempera- 

 ture. — When pyrite is undergoing oxydation, through the decomposition of water> 

 the hydrogen of the decomposed water will form sulphid of hydrogen with the sulphur, 

 and so give origin to "sulphur springs." This sulphid of such springs may also be- 

 come oxydized, the sulphur making with the oxygen sulphuric acid, and the hydrogen 

 producing wafer, and thus may be produced sulphuric acid springs ; though this acid is 

 so strong in its affinities that it seldom is allowed to remain free. 



C. Effects at an Elevated Tempehature. — In volcanoes, the vapors of water, 

 in connection with sulphur vapors from sulphids, give origin to sulphurous acid or sul- 

 phuretted hydrogen ; and the acid is destructive to the volcanic rocks within its reach. 



D. Effects through the Dissociated Elements of Water. — At temperatures 

 about 1800° F., the elements of water are separated. In the process of metamorphism, 

 this temperature has, beyond doubt, been sometimes concerned. But, as the moisture 

 present was under high pressure, and pressure raises the temperature of dissociation, it 

 is not certain that this means of change has been an actual one, at least since the earth's 

 crust was first formed. 



3. Destructive Effects through or by the aid of Sub- 

 stances held in Solution in Water. 



A. Carbonic Acid. — The most important agent of destruction, as 

 well as of construction, among the substances dissolved in water, is car- 

 bonic acid gas ; and it starts for its work mostly from the atmosphere, 

 although constituting but four parts in 10,000 of air. In Archaean 

 time, as stated on page 156, its effects were far greater than now, owing 

 to the much larger proportion of carbonic acid in the atmosphere ; and 

 from that time they have gradually diminished. It is carried from the 

 air to the earth's rocky surface in all precipitated moisture, and is con- 

 sequently present in all streams, lakes, and oceans. Other prominent 

 sources of this gas in the earth's waters, and in the soil, are : (1) the 

 respiration of aquatic and underground animals, carbonic acid con- 

 stituting a large part of the air exhaled ; (2) vegetable and animal 

 decomposition, carbonic acid being an ultimate product, as it is of the 

 combustion of coal ; (3) chemical agents (mentioned beyond), sepa- 

 rating carbonic acid from carbonate of lime. 



1. Eroding Action. — Carbonic acid has a strong affinity for potash, 

 soda, lime, magnesia, and iron. If waters containing carbonic acid are 

 made to pass through powdered feldspar, mica, hornblende, pyroxene, 

 limestone, and other mineral materials containing these substances, 

 portions of them will be taken up and carried off; and the disorgani- 

 zation thus begun is attended by a loss also of silica and alumina, and 

 ends in the destruction of the rock made of these minerals, so far as it 

 is subjected to the process. Professors W. B. and R. E. Rogers found, 

 in their experiments on the action of carbonated waters, 0-4 to one per 

 cent, of the whole mass under digestion dissolved away in only forty- 

 eight hours. 1 



# 1 American Journal of Science, II., v., 401. 

 45 



