STATES OF THE GASES. 35 



expel the gaseous constituents of the rocks, high pressures would have the 

 effect of promoting absorption. Moreover, it is possible that molten 

 lavas might absorb, or dissolve, certain gases without an increase of pres- 

 sure. But the testimony of volcanic gases arid of the scoriaceous surfaces 

 of lava flows favors the idea that gases and vapors are constantly being 

 boiled out of molten lavas whenever exposed under the ordinary atmo- 

 spheric pressure. Lavas give off gas rather than absorb it, at the earth's 

 surface; however, at considerable depths below the surface the action may 

 be entirely different. If the conception be entertained that the earth's 

 interior is, for the most part, solid with only threads of liquid lava here 

 and there, the question for this solid portion would be one of the ability 

 of great pressure to cause a solid to absorb gases. This need not be further 

 dwelt upon, since most of the igneous rocks which are accessible have been 

 in the liquid state at some time. In the case of the threads of liquid magma 

 there is reason to suppose that gas, if it could be brought into contact 

 with this lava, would become incorporated in it owing to the great pres- 

 sure. But this does not explain the original source of the gases, nor how 

 they can be brought in contact with the liquid rock under the prevailing 

 conditions of temperature and pressure. 



STATES IN WHICH THE GASES EXIST IN ROCKS. 



In order to explain the immediate source of the gases obtained by 

 heating rock material in vacuo, three different hypotheses naturally pre- 

 sent themselves. The simplest of these is to suppose the gases to exist in 

 minute cavities or pores, having been entrapped within the rock during the 

 process of solidification. This supposition is suggested and supported by 

 the observation that microscopic slides of some minerals, notably quartz 

 and topaz, reveal numerous small gas-bubbles. But while there is evidence 

 that some gas is thus held in cavities, there is equally strong evidence to 

 show that the greater part of it can not be attributed to this source. 



To escape the difficulties encountered by the first hypothesis, appeal 

 is made to the imperfectly understood property of some of the elements 

 to "occlude," or dissolve within their mass, certain gases. It is remem- 

 bered that under the proper conditions palladium will occlude 900 times 

 its own volume of hydrogen, and that the same gas is also absorbed, in 

 lesser degree, by other metals, particularly platinum 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 re- 

 actions at high temperature. The oxides of carbon and sulphur are assigned 

 to the decomposition of carbonates and sulphates; methane to organic 

 matter present, carbides, or to high temperature reactions between hydro- 

 gen and the carbon gases; sulphureted hydrogen to sulphides; nitrogen 

 to nitrides; while hydrogen is liberated from steam by the action of metallic 

 iron or ferrous salt. 



