76 THE GASES IN ROCKS. 



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 of the volcanic activity. A source for carbon dioxide 

 after the disappearance of the other gases has been sought in the neigh- 

 boring 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 continue for some time after the volcano has settled into 

 quiescence. Some of this carbon dioxide doubtless also comes from previ- 

 ous lavas which, warmed again by the fresh lava, give up some of the carbon 

 dioxide which my experiments show them to contain. 



AMMONIUM CHLORIDE DEPOSITS. 



Among the various substances which are deposited around fumaroles, 

 sal-ammoniac, or ammonium chloride, is, in some respects, one df the most 

 remarkable. Compounds of ammonium have not yet been recognized in 

 igneous rocks, although rock powders often give off small quantities of 

 ammonia gas when heated in vacuo. Chemical analyses of spring-waters 

 report ammonium salts only in traces, such as may have been derived 

 from the decay of organic matter. If ground-waters be, for the most part, 

 unable to reach the lavas, even this rather doubtful source of ammonium 

 compounds is not available. If the elements of the radical NH 4 be supposed 

 to have come from the interior magma, there are two alternative hypotheses 

 still open. The first assumes that the radical NH 4 existed intact in the 

 magmatic solution in the form of ammonium salts and, volatilized by the 

 heat upon the relief of pressure, gradually collected on the cooler portions 

 of the crater. This hypothesis must, however, explain the apparent absence 

 of these compounds in igneous rocks. The second believes that the am- 

 monium chloride was formed synthetically in the throat of the volcano, 

 from the nitrogen, hydrogen, and hydrochloric-acid gases. This would make 

 it a direct product of volcanic gases. 



The presence of ammonia, or its vaporized salts, in volcanic emana- 

 tions leads to the formation of another interesting compound. Silvestri l 

 has found iron nitride, as a lustrous metallic deposit, at a fumarole on 

 Etna. This compound is due either to a reaction between the sublimed 

 ferric chloride and free ammonia gas or to the ignition of the iron-bearing 

 lava in the presence of ammonium chloride vapor. The appearance of iron 

 nitride around fumaroles throws no direct light upon the question of its 

 existence in the magmas, though it indirectly leads to the hypothesis that 

 the nitrogen in the ammonia and its compounds came originally from iron 

 nitride within the magma. 



1 Silvestri, Pogg. Ann., vol. 157 (1876), pp. 165-172. 



