60 A TREATISE ON METAMORPHISM. 



to the action of some particular compound or compounds, or else to some 

 form of compound, such as gases. 



Gaseous solutions require consideration from two points of view — the 

 chemical and physical principles controlling the action of gases and the 

 ffeolog'ical work of erases. 



SECTION i. CHEMICAL AND PHYSICAL PRINCIPLES CONTROLLING THE ACTION 



OF GASES. 



The chemical and physical principles controlling the work of gases 

 may be considered under (1) the gases present, (2) the pressure, and (3) 

 the temperature. 



Gases present. — The law of greatest importance controlling the chemical 

 action of gaseous solutions is: The properties of a homogeneous mixture or 

 solution of various gases are the sum of the properties of the constituents of 

 the mixture. To illustrate, when carbon dioxide (CO,) and oxygen (0 2 ) are 

 mixed the properties and activities of each are the same as if the same 

 quantity of each were free from the other and occupied the same space. 

 Therefore, in the belt of weathering, where gases are active, the carbon 

 dioxide and oxygen are both doing their work, the one that of carbonation 

 (see pp. 473-480), the other that of oxidation (see pp. 461-473), as if the 

 other were not present. It is clear, therefore, that the properties of gaseous 

 mixtures are additive. 



Slight deviations from this law have been noted under certain condi- 

 tions, but these mainly concern the exact physics of the gaseous solutions 

 rather than their geological work, and hence are not here considered. In 

 applying the law, however, we must be sure that the gases do not unite 

 chemically and produce a new compound. To illustrate, while the law is 

 certainly applicable to the case mentioned, that of a mixture of carbon 

 dioxide and oxygen, it is not certain that this is the case when water gas 

 (H 2 0) and carbon dioxide or sulphurous oxide (S0 2 ) are mixed, for these 

 compounds may unite with water gas, producing carbonic acid (H 2 C0 3 ) 

 and sulphurous acid (H 2 S0 3 ) gases. Certainly the law will not apply to a 

 mixture of the gases ammonia (NH 3 ) and water, for these gases will largely 

 unite and produce ammonium hydroxide (NH^OH), which may exist in the 

 form of gas. In case a gas be formed by the union of two or more gases, 



