STATES OF THE GASES. 49 



which was dug up while still hot, gave Stokes 0.25 per cent of water. 1 

 Perhaps this was moisture absorbed from the air by deliquescent com- 

 pounds, such as lawrencite; still, on the other hand, there appears no 

 reason, at the present time, why a part of this water should not be a pri- 

 mary constituent of the meteorite. This uncertainty points out the desir- 

 ability of further, and more critical, studies upon the composition and 

 properties of meteorites, before attempting to base an argument upon the 

 absence of water in these bodies. 



Other possible sources of hydrogen are hydrogen sulphide, hydro- 

 carbons, and the products of radioactivity. As the decomposition of 

 sulphureted hydrogen has already been mentioned, and is also treated 

 under the head of that gas, it need not be discussed here. Hydrocarbons 

 can only be represented in small quantities in igneous rocks, and should 

 produce more methane than free hydrogen. Unless the analysis shows 

 much marsh-gas, hydrogen from this source must be unimportant. 



CARBON DIOXIDE. 



The carbonates of most metals are decomposed by heat with the liber- 

 ation of carbon dioxide. On this account the determination of the carbon 

 dioxide yielded by rocks which have undergone much carbonation is of 

 little value. Many rocks which appear to be perfectly fresh have neverthe- 

 less suffered slight carbonation while in the zone of weathering, and thus 

 possess carbon dioxide in a combined state ready to be evolved when suffi- 

 ciently heated. This carbon dioxide from the non-gaseous constituents 

 of the rock embarrasses the determination of the free gas, since there is no 

 way of separating the carbonic acid from these different sources. 



The degree of heat necessary to decompose carbonates throws some 

 light on the question. Erdmann and Marchand state that already at 

 400 traces of carbon dioxide are given off from calcium carbonate. 2 The 

 studies of Debray show that at the boiling-points of mercury and sulphur, 

 350 and 448 respectively, the development of CO 2 from calcite in vacuo 

 is inappreciable. 3 The same investigator found that at 860 calcite gives 

 up carbonic anhydride until a pressure of 85 millimeters is reached, when the 

 action ceases. At 1040 the pressure may rise to 520 millimeters before 

 the evolution of gas is stopped. In the presence of carbon dioxide at the 

 ordinary atmospheric pressure, calcite retains all of its optical and other 

 properties unaltered, even at 1040. Carbon dioxide from calcium carbonate 

 is thus not of any quantitative importance below 450. In general, most 

 of the carbonic acid from the rocks is expelled at temperatures above 

 450. But considerable CO 2 often appears before the heat reaches 400, 

 as is shown by the Baltimore gneiss. Perhaps this gas may be assigned to 

 ferrous carbonate. Iron carbonate would be expected to decompose more 

 readily than calcium carbonate, though I have been unable to discover at 

 what temperature the process commences. 



1 Ante, p. 21. 



2 Erdmann and Marchand, cited by Gmelin-Kraut, Anorg. Chem., 2, p. 354. 



3 Debray, Comptes Rendus, vol. 64, p. 603. 



