STATES OF THE GASES. 57 



in limited quantities under the quite exceptional conditions of abundant 

 currents of free gases and very active hot vapors, to the main magmas. 



If the nitrogen obtained from rock powders be derived from a nitride, 

 it should be accompanied by ammonia, since, in the presence of hydrogen 

 or water-vapor, it is this gas, rather than free nitrogen, which is given off. 

 Tests made with Nessler's solution show that ammonia is one of the gases 

 extracted from rocks, though always appearing in limited amounts. In 

 the process ordinarily employed for extracting the gas, it is absorbed by 

 the calcium chloride drying-tube. Ammonia is scarcely to be considered 

 as a source of free nitrogen, since this compound is only dissociated at the 

 temperature of the electric spark. 



Whether all of the free nitrogen can be assigned to the decomposition 

 of iron nitride may be tested with the quartz from New South Wales. 1 Sup- 

 posing all of the iron in this quartz to have existed as iron nitride, and 

 to have been completely decomposed without the production of any am- 

 monia, the analysis still shows an excess of nitrogen over what could have 

 been produced in this way. The reaction may be taken as 



102.72 gms. quartz contained ............................ 0.0058 gm. Fe 



Fe (as Fe 2 N) required to give 1 c.c. nitrogen ................ 0100 gm. 



Nitrogen possible from reaction ........................... 58 c.c. 



Nitrogen actually obtained (0 and 760 mm.) ............... 86 c.c. 



Excess of nitrogen ....................................... 28 c.c. 



A duplicate determination of the iron in this weight of quartz gave only 

 0.0048 gram; on this basis, the excess of nitrogen would be still greater. 

 It is highly improbable that all of the iron in this quartz was combined as 

 a nitride. Some of it was unquestionably pyrite. To ascertain how much 

 of this nitrogen can be ascribed to atmospheric air adhering to the tubes, 

 as well as to leakage during the process of extraction, a blank combustion 

 was resorted to. The empty combustion-tube was kept at bright-yellow 

 heat for the length of time which was required to expel the gas from the 

 quartz. 0.15 cubic centimeter of gas was collected in the receiver when 

 the tube was exhausted by the pump. Adhesion of air to the quartz itself 

 might be supposed to increase this figure, though the material used for this 

 analysis was not the usual fine powder, but small fragments which would 

 be less liable to entrap air. In general, while iron nitride is to be accepted 

 as a possible source of this nitrogen, it is inadequate to produce the quanti- 

 ties of this gas determined by analysis. The presence of other metallic 

 nitrides in this comparatively pure quartz does not seem likely. Silicon 

 nitride, however, may be present and might possibly contribute a portion 



of the nitrogen. 



OCCLUDED GASES. 



Though occlusion is a phenomenon but imperfectly understood, there 

 appear to be three different ways in which it is manifested. In the first 

 of these the absorption seems to be dependent upon porosity. An example 

 of this is charcoal, one variety of which absorbs 172 volumes of ammonia, 



1 Analysis No. 100. 



