36 



THE GASES IN ROCKS. 



GASES IN CAVITIES. 



The studies of Brewster, Davy, Sorby, Hartley, and others, have 

 established the presence of gas, generally carbon dioxide, though sometimes 

 nitrogen, in the minute cavities of certain crystals. This has been widely 

 known to geologists, and hence, when it was discovered that many crystal- 

 line rocks yield gas upon heating in vacuo, it was natural to suppose that 

 the gas came from cavities. Such was the view taken by Tilden. 1 But 

 while microscopical investigations indicated that carbon dioxide consti- 

 tutes more than 90 per cent of the gaseous matter inclosed in these cavi- 

 ties, and hydrogen is not found in more than traces, the latter gas is the 

 most important constituent of the mixture derived from rocks by heat. 

 In addition to this, the observation that those rocks which are not known 

 to contain many gas cavities produced several times as much gas as the 

 cavernous quartzes also suggested that the bulk of the gas, at least, could 

 not be attributed to inclosure in cavities. Moreover, basic rocks were 

 found to be more productive than acidic, whereas it had generally been 

 supposed that the latter, owing to their greater viscosity, should entrap 

 more gas and vapor than the more fluid basic lavas. 



The suspicion that the gas did not come from cavities in any large 

 degree was strengthened by the observation that the composition of the 

 gas varied according to the temperature to which the rock powder was 

 heated. If the gas comes from cavities, its liberation should commence 

 with a slight rise of temperature and should continue more or less steadily, 

 as the heat increases, until the expansive force of the gas opens up most 

 of the pores. Since all gases expand equally, one should burst its con- 

 fines as soon as another, and a sample of gas obtained at any given 

 temperature should not differ very widety in composition from that evolved 

 at any other. 



Neglecting hydrogen sulphide and nitrogen, the character of the gas 

 obtained at various temperatures from Baltimore gneiss 2 is shown by the 

 following table: 



TABLE 24. 



Or, combining the separate analyses so that each figure represents the 

 percentage of the total gas obtained up to the specified temperature, the 

 result is as shown in table 25 : 



1 Tilden, Chem. News, vol. 75 (1897), p. 169; Proc. Roy. Soc., vol. 64 (1897), p. 453. 



2 Material of analysis, No. 28. 



