66 THE GASES IN ROCKS. 



A difficulty of a more serious nature, apparently, suggested by Professor 

 Iddings, lies in the change of volume of the magma in the passage from the 

 liquid to the crystalline form. Some magmas, such as those of granitic 

 rocks, contract so appreciably upon crystallization that it is conceivable 

 that the last crystals to form, those of quartz (which also contain the most 

 liquid and gas inclusions) might crystallize under reduced pressures in 

 spaces inclosed by crystals of the minerals already formed. The relative 

 size of the bubble of vapor in the cavity and the accompanying liquid 

 would, in such cases, not correspond directly to the depth beneath the 

 surface at which crystallization took place, even when nothing but hydro- 

 static pressure affected the lava column. 



In the present defective state of knowledge as to the modes and condi- 

 tions which obtain in lavas penetrating the shell of the earth, it is by no 

 means safe to assume that the pressures to which an igneous intrusion is 

 subject are merely those represented by the overlying rock or a lava 

 column reaching to the surface. An ascending tongue of lava may extend 

 to great depths and be affected by pressures brought to bear upon it in its 

 lower part, which might be in excess of those represented by the depth of 

 the head of the column, to an unknown degree. So also it is possible that 

 lavas may become involved in mechanical deformations and thus be subject 

 to special pressures in no close correspondence to their depth. 



WATER AND HYDROGEN. 



The reversible reactions involving hydrogen, water, and iron com- 

 pounds, which cause uncertainties in the extraction of gases by heat, 

 are also operative within the earth. In the laboratory, when either ferrous 

 salts and water, or ferric compounds and hydrogen, are heated in tubes 

 without the removal of the products, reversible reactions set in until a 

 condition of equilibrium is established. Hydrogen and water, ferrous and 

 ferric salts are all present in a state of balance. In the interior of the earth 

 the heated, though solid, rocks should, it would seem, behave similarly, 

 though hindered by the slowness of diffusion. Nor should liquid magmas 

 constitute any exception to the law. Both hydrogen and water-gas, theo- 

 retically, should be present in liquid magmas and heated solid rocks. The 

 chief uncertain factors are high temperatures, and pressures. 



The effect of pressure on chemical equilibrium is to favor the formation 

 of that system which occupies the smaller volume, but if there is no change 

 in volume, in passing from one system to the other, the increase of pressure 

 presumably has no influence on equilibrium. 1 In the reaction 



3FeO+H 2 O ^~7 Fe 3 O 4 + H 2 



considered as a thermochemical equation, the number of gaseous mole- 

 cules, and hence the volume of gas, always remains the same, so that it 

 is not likely that this action will be influenced by change of pressure. A 

 rise of temperature favors the formation of that system which absorbs 

 heat when it is formed. 2 A comparison of the amount of heat liberated by 

 oxidizing three molecules of FeO to Fe 3 O 4 and one molecule of H 2 to H 2 O 



1 Jones, Physical Chemistry, p. 514. 



2 Van't Hoff, Lectures on Theoretical and Phys. Chem., Pt. 1, pp. 161-164. 



