THE GASES IN ROCKS 549 
num and iron, while silver has a similar affinity for oxygen. This 
principle applied to igneous rocks as a hypothetical source of their 
gases becomes at once a more difficult proposition to prove or disprove. 
The third hypothesis, more conservative than either of the others, 
assumes that these gases do not exist in the rocks in the uncombined, 
or gaseous state, but are produced in the combustion-tube by chemical 
reactions at high temperature. 
GAS IN CAVITIES 
The studies of Brewster, Davy, Sorby, Hartley, and others, have 
established the presence of gas, generally carbon dioxide, though 
sometimes nitrogen, in the minute cavities of certain crystals. But 
while microscopic investigations have indicated that carbon dioxide 
constitutes more than go per cent. of the gaseous matter inclosed in 
these cavities, and hydrogen is found in traces only, the latter gas is 
the most important constituent of the mixture derived from rocks by 
heat. In addition to this, the observation that those rocks which are 
not known to contain many gas cavities produced several times as 
much gas as the cavernous quartzes, also suggested that the bulk of 
the gas, at least, could not be attributed to inclosure in cavities. 
Moreover, basic rocks were found to be more productive than acidic, 
whereas it had generally been supposed that the latter, owing to their 
greater viscosity, should entrap more gas and vapor than the more 
fluid basic lavas. 
The suspicion that the gas did not come from cavities in any large 
degree was strengthened by the observation that the composition of 
the gas varied according to the temperature to which the rock powder 
was heated. If from cavities, the liberation of the gas should com- 
mence with a slight rise of temperature, and should continue more or 
less steadily, as the heat increased, until the expansive force of the 
gas had burst open most of the pores. Since all gases expand equally, 
one should burst its confines as soon as another, and a sample of gas 
obtained at any given temperature should not differ very widely in 
composition from that evolved at any other. 
These and other considerations led me to try a series of experiments 
which should show how much gas actually could be obtained from 
the opening of cavities alone. For this purpose a crusher was devised, 
