THE GASES IN ROCKS 565 
dioxide, and nitrogen. Carbon dioxide and nitrogen escape from all 
the fumaroles. Fouqué found that the relative importance of hydro- 
gen increased with rise of temperature, and that his marsh-gas (which, 
owing to an imperfection in the method of analysis in 1867, may have 
been carbon monoxide, or a mixture of carbon monoxide and marsh- 
gas) diminished as the activity increased. ‘These observations are 
entirely in accord with the results of my differential temperature 
experiments with rock powders. Hydrogen sulphide and carbon 
dioxide are the gases expeiled from the rocks at the lowest tempera- 
tures; carbon monoxide and marsh-gas appear at intermediate 
temperatures, while hydrogen is most prominent when the heat is 
carried to bright redness. Nitrogen is most abundantly liberated at 
red heat; hence the presence of that gas at the cooler vents and fissures 
is chiefly due to atmospheric air. 
While carbon dioxide escapes from all fumaroles in greater or less 
degree, it is at those vents whose activity has subsided beyond the— 
point where hydrogen and the noxious gases are evolved that this gas 
is most conspicuous. For this reason, carbon dioxide has come to 
be regarded as marking the dying-out of the volcanic activity. A 
source for carbon dioxide after the disappearance of the other gases 
has been sought in the neighboring limestone formations, either from 
baking or from the chemical action of halogen or sulphur acids. The 
obvious difficulty confronting this conception is that limestone is not 
always present to furnish carbon dioxide. Experiments show that 
below 400° C. carbon dioxide is the principal gas evolved from rock 
material, and as the lava solidifying in the crater, or conduit, has not 
lost all its gas, it is only a part of the natural sequence of events that 
the escape of carbonic anhydride from the cooling lavas should con- 
tinue for some time after the volcano has settled into quiescence. Some 
of this carbon dioxide doubtless also comes from previous lavas which, 
warmed again by the fresh lava, give up some of the carbon dioxide 
which my experiments show them to contain. 
GENERAL RELATIONS 
RELATIVE TO THE HYPOTHESIS OF A MOLTEN EARTH 
These studies show that, within the range of temperature employed, 
heat causes the expulsion of gases in whatever form they are held, and 
