58 THE GASES IN ROCKS. 



165 volumes of hydrogen chloride, 97 volumes of carbon dioxide, and 2 

 volumes of hydrogen. 1 The affinity of molten silver for oxygen illustrates 

 another phase of absorption often classed as occlusion. It has long been 

 known that silver absorbs 22 times its own volume of oxygen when melted, 

 but gives up most of this gas, often with violence, as it solidifies. 2 This 

 is properly a solution of a gas in a liquid, and not in a solid, as in the case 

 of true occlusion. The third type is the absorption of gases by compact 

 metals, either on their surface or within their mass, such as the occlusion 

 of hydrogen by palladium, platinum, and iron. This is, in the main, inde- 

 pendent of porosity. 



Hydrogen is absorbed by these metals at ordinary temperatures, but 

 is only given off at higher temperatures. This principle was demonstrated 

 by Graham, who placed a thin plate of palladium, charged with hydrogen. 

 in a vacuum and observed that at the end of two months the vacuum was 

 still perfect. No hydrogen had vaporized in the cold, but on the applica- 

 tion of a heat of 100 and upwards, 333 volumes of gas were evolved from 

 the metal. 3 The degree of heat required to expel hydrogen absorbed by 

 platinum and iron was found to be little short of redness, although the 

 gas had entered the metal at a low temperature. Another series of experi- 

 ments by the same investigator showed that, to be occluded by palladium 

 and even by iron, hydrogen does not need to be applied under sensible 

 pressure, but on the contrary, when highly rarefied, it is still freely absorbed 

 by these metals. These results have been confirmed by Mond, Ramsay, 

 and Shields, 4 who found that platinum black at very low pressures absorbed 

 a certain quantity of hydrogen. On increasing the pressure of the hydro- 

 gen up to about 200 to 300 millimeters, a further quantity was absorbed, 

 but beyond this point an increase of pressure had comparatively little 

 effect. These investigators regarded 110 volumes as the amount of hydro- 

 gen really occluded by platinum black, although 310 volumes were actually 

 absorbed. 



Experiments indicate that the quantity of hydrogen occluded depends 

 greatly upon the condition of the metal. When chemically reduced, cobalt 

 may occlude 59 to 153 volumes, nickel 17 to 18, and iron 9 to 19 volumes. 5 

 Though common iron wire occludes only 0.46 volume of hydrogen, 6 this 

 same metal, when electrolytically deposited, may absorb nearly 250 vol- 

 umes of this gas. 7 The maximum quantity of hydrogen occluded by any 

 metal, so far as recorded, is 982 volumes absorbed by freshly precipitated 

 palladium. 8 Dumas has shown that aluminum heated in vacuo to 1400 

 gives off more than its own volume of gas, consisting chiefly of hydrogen 

 with a little carbon monoxide, but without traces of carbon dioxide, oxygen, 

 or nitrogen. 9 Under the same conditions, magnesium rapidly expels 1.5 



1 Barker, Textbook of Physics, p. 183. 



2 Chimie Minerale, Moissan, t. 1, p. 203. 



3 Graham, Chemical and Physical Researches, pp. 283-290. 



4 Proc. Roy. Soc., vol. 58 (1895), pp. 242-243. 

 8 Chimie Minerale, Moissan, t. 1, p. 51. 



6 Graham, Chemical and Physical Researches, p. 279. 



7 Cailletet, L'Institut, Nouv. Se>., Ann. 3, p. 44. 



8 Graham, Chemical and Physical Researches, p. 287. 



9 Dumas, Comptes Rendus, 90 (1880), p. 1027. 



