PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 617 
clusion can be drawn as to the nature of the gaseous phase present 
in the intrusive magma unless the conditions existing there and 
their influence upon the equilibrium are all known. Nay more, 
it is to be expected that the exhaled gases do not represent an 
equilibrium at all, but are actively in the process of reacting." 
The magma also contains, in addition to the above gases and 
vapors, a number of other substances which are volatile at high 
temperatures, such for example as metallic sulphides, chlorides 
or fluorides, and compounds containing boron. 
It is not at all necessary that the gaseous components, such as 
H.O, H.S, HCl, SO.,, exist as gases in the magma, even when they 
are present in the free state and not as compounds. For in that 
case it is their miscibility with the magma—or in other words, their 
solubility in it under the particular conditions (of which pressure 
is obviously the most important)—which determines whether they 
are present in the liquid phase. 
The critical temperature, above which a system can exist only 
as gas, is a function of the chemical composition of the system. 
A gas when dissolved in a mixture of non-volatile components will 
exhibit a higher critical temperature than it does in the pure state; 
such a solution would of course exert in general a considerable 
gas pressure, which, however, need not be as great as the critical 
pressure of the volatile component in the pure state.? To illus- 
trate: the critical temperature of SO, is 157°5, its critical pres- 
sure is 79 atm. Now if 24.7 parts HgBr, (the melting-point of 
which is 236°5) and 75.3 parts SO, are heated in a sealed glass 
tube, the volume of which is so chosen that the pressure will be 
about 80 atm. at 157°, melting of the HgBr, takes place at 230°3 
This lowering of 6°5, which is due to the taking-up of SO, by the 
* Day and Shepherd, for example (in the course of an investigation as yet unpub- 
lished), find that the gases emerging from the volcano Kilauea contain CO, CO:, S, 
SO., H.0, H, Cl, and F—a mixture of gases which cannot possibly coexist in equi- 
librium. 
2 A full discussion of these matters will be found in P. Niggli, Z. anorg. Chem., 
LXXV, 161; LXXVII (1912), 321; Central. Min., 1912, p. 321; Geol. Rundschau, II 
(1912), 472. The main points, especially in case the volatile component is water, are 
treated by Morey and Niggli, Jour. Am. Chem. Soc., XX XV (1013), 1086 f. 
3Z. anorg. Chem., LXXII (1912), 161 f. 
