WATER AS A CHEMICAL AGENT. 689 



erous carbonate of lime, or carbonate of lime and magnesia (in which 

 iron replaces part of the calcium or magnesium), undergoes the same 

 kind of destruction, though less rapidly. The rock often becomes re- 

 duced to a bed of more or less pure limonite. Crystalline limestones 

 usually undergo this change more readily than common massive lime- 

 stone, because more permeable to moisture. 



3. Other Cases of Oxydation. — The oxydation of carbon, hydrogen, and other in- 

 gredients of vegetable and animal matter, is another important means of geological 

 change, through the oxygen of water and air. The fallen unburied leaves and stems 

 of the forest have their carbon changed by this means to carbonic acid, and so, in a 

 true sense, consumed; and if buried, the air being to a great extent excluded, part of 

 the carbon will be preserved to make coal, while other portions will be lost by this sort 

 of combustion (p. 363). Animal matters are subject to an analogous change. 



On the evaporation of water from a moist surface, or after a rain, it has been ob- 

 served that there is a production of ozone over the surface, and, Schonbein says, 

 of nitrite of ammonia. Whatever the chemical effects of such a cause, they must be 

 of wide influence. But, while water is essential to the result, the ozone is probably 

 from the oxygen of the air present. In the production of nitrates in covered places or 

 caverns, the same production of ozone has been supposed to be a step, it leading to the 

 oxydation of the nitrogen of the atmosphere, or of organic substances present. The 

 production of such nitrates has considerable mechanical effect, in disintegrating the 

 outer portion of loosely aggregated rocks in covered places ; and there must be chemical 

 effects besides, yet to be studied. 



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 ivater, 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 Temperature. — 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,asthe 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- 

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