HISTORICAL SKETCH. 



9 



not be found to enter into combination at such temperatures as are ordi- 

 narily attainable. In his own words: 



It also appears improbable that in the minerals from which the mixture of gases con- 

 taining helium has been extracted this element exists in a state of ordinary chemical com- 

 bination, for, on treating the mineral with acids, no compound of helium with hydrogen has 

 yet been observed, and the components of the minerals are of a kind which are commonly 

 regarded as chemically saturated. 1 



The minerals monazite and cleveite were found to yield gas at low tem- 

 peratures (60 and 110, respectively), carbon dioxide appearing first. The 

 monazite heated to 130 to 140 gave gas which, for the first time, showed 

 the D 3 line, indicating the presence of helium. Between 140 and 250 

 there was obtained carbon dioxide with about one-fourth of its volume of 

 a gas rich in helium. At higher temperatures up to 446 (boiling sulphur) 

 there was less gas evolved. Cleveite behaved in a similar way. Studies 

 on the absorption of helium by cleveite demonstrated that the mineral 

 does not absorb this gas at the ordinary pressure, although placed in a 

 helium atmosphere for nine weeks. But under pressure of 2.5 and 7 atmos- 

 pheres, Tilden believed that he obtained an appreciable absorption. A 

 trial with the Peterhead granite, which contained no helium in the first 

 place, proved that the granite would absorb none of the gas whatever, 

 even aided by a pressure of 7 atmospheres. 



The finding of hydrogen as well as carbon dioxide in this Peterhead 

 granite 2 led Tilden to investigate the gases inclosed in crystalline rocks. 3 

 His five complete analyses are as given in table 5. 



TABLE 5. 



In addition to these analyses, 25 carbon-dioxide determinations were 

 made. 



Tilden believed the gas to be " wholly inclosed in cavities which are 

 visible in thin sections of the rock when viewed under the microscope. 

 * * * To account for the large proportion of hydrogen and carbon 

 dioxide in these gases, it is only necessary to suppose that the rock inclos- 

 ing them was crystallized in an atmosphere rich in carbon dioxide and 

 steam, which had been, or were at the same time, in contact with some 

 easily oxidizable substance, at a moderately high temperature. Of the 

 substances capable of so acting, carbon, a metal, or a protoxide of a metal 

 present themselves as the most probable." Hydrogen and carbon monox- 



1 W. A. Tilden, Proc. Roy. Soc., vol. 59 (1896), p. 218. 



2 Wright's two analyses, showing that trap rocks yield much the same gases as meteor- 

 ites, also served to call attention to this field for investigation. 

 3 Tilden, Chem. News, vol. 75 (1897), pp. 169-170. 



