STATES OF THE GASES. 41 



Being desirous of finding some specimen which would yield gas when 

 crushed in this manner, I procured some crystals of cavernous quartz 

 from Porretta, Italy, in which several of the cavities exceeded a millimeter 

 in diameter. 5.91 grams were crushed to sufficient fineness to pass through 

 the sieve, and 61.66 grams were partially crushed. 0.08 cubic centimeter 

 of carbon dioxide was obtained, which, supposing that it all came from the 

 5.91 grams, would be equivalent to only 0.03 of the volume of the quartz. 

 An analysis showed also a little methane and some nitrogen, but the amount 

 of gas available was too small for the determination to be of any value. 



The result of this last test agrees with the microscopic studies of the 

 early investigators. Carbon dioxide exists in the cavities of quartz, but 

 its volume, compared with the volume of the inclosing mineral, is small. 

 Microscopical observations seem to show that gas cavities occur almost 

 exclusively in a certain set of minerals which combine hardness usually with 

 imperfect cleavage, namely, quartz, topaz, garnet, spinel, beryl, chrysoberyl, 

 corundum in the form of rubies, sapphires, and emeralds, and diamond. 

 These are minerals which, once they had inclosed gas, would hold it, even 

 under great pressure. 



GASES DUE TO CHEMICAL REACTIONS. 

 HYDROGEN. 



The double series of iron salts, ferrous and ferric, together with the 

 intermediate ferroso-ferric compounds, reacting with oxidizing or reducing 

 agents, undergo various reversible reactions whose possibilities are great. 

 When steam is passed over metallic iron or ferrous oxide at a red heat, it 

 is decomposed, giving up oxygen to the iron, and at the same time pro- 

 ducing free hydrogen. The reactions may be written: 



Fe + H 2 = FeO + H 2 3FeO + H 2 O = Fe 3 O 4 + H 2 



Hydrogen is produced in this way most rapidly at temperatures about 

 500. Stromeyer is authority for the statement that the breaking up of 

 water begins at 150 but takes place very slowly; at 200 somewhat more 

 rapidly; at 360 the process requires several hours; at 860 it is complete 

 in less than one hour; while near the melting-point of iron several minutes 

 are sufficient. 1 



The authorities agree that ferric oxide is not formed in this process; 

 the magnetic oxide, Fe 3 O 4 is the final product of the action of a current 

 of steam upon ferrous oxide. 2 



But these reactions are completely reversible. According to Gay- 

 Lussac, magnetite is reduced to the metal by hydrogen at every tempera- 

 ture between 400 and the highest degree of heat obtainable in the com- 

 bustion-furnace, particularly at the same temperature at which steam is 

 split up by glowing iron. 3 Siewert states that ferric oxide (from the oxa- 

 late) is not altered by hydrogen at 270 to 280; between 280 and 300 



1 Stromeyer, Pogg. Ann., vol. 9, p. 475. 



2 Among others, Regnault, Ann. de Chim. et Phys., vol. 62, p. 346. 



3 Gay-Lussac, Ann. de Chim. et Phys., vol. 1, p. 33. 



