Chap. 12] 



MISCELLANEOUS GEOPHYSICAL METHODS 



897 



does not differentiate between the hydrocarbons contained in the soil air 

 and gives greater indications for the heavier constituents. 



Difl&culties experienced with instability of the capillary induced Sokolov 

 to design another combustion instrument, illustrated in Fig. 12-12. Its 

 action is based on a determination of the amount of air which must be 

 burned to liberate a minimum detectable volume of carbon dioxide. De- 

 tection is made by the turbidity produced by the passage of the gas 

 mixture through a barium hydroxide solution after combustion. A carbon 

 dioxide content of about 0.03 per cent is required to cause turbidity. The 

 corresponding vohime of air may be read on a burette and is called the 

 apparatus constant C. If air containing hydrocarbons is then burned 

 and carbon dioxide is liberated, and if F is the volume of air required to 



Fig. 12-12. Portable (combustion-absorption) gas detector (after Sokolov). 

 (a) Caustic potash tubes for removal of carbon-dioxide; (6) burette; (c) combustion 

 tube; (d) absorption capillary; {I) leveling flask. 



start turbidity, then the content of carbon dioxide in percentage is 

 0.03 X C/V. The smaller the hydrocarbon content, the greater there- 

 fore is the volume required to cause turbidity. Hence, it is a virtue of 

 this apparatus that the readings increase as the hydrocarbon content 

 decreases. It is obvious that the air must be well cleaned of carbon 

 dioxide before being injected into the apparatus. For this purpose the 

 two bubble tubes a filled with caustic potash are provided. In applica- 

 tion, the soil air is first passed through these tubes, and through the com- 

 bustion chamber c (cold filament), the turbidity capillary d (constructed 

 like the Hankus type of Orsat pipette), and the burette h. Then the 

 capillary pipette d is filled from a jar with (iV^/10) Ba(0H)2 solution, the 

 filament turned on, the stopcocks closed, and the burette read. As soon 

 as turbidity appears, the burette is read again, the filament turned off, 

 and the capillary tube washed. The apparatus is calibrated by known 

 volumes of CO? or CH4 and is reported to be accurate to 1 • 10~* to 1 • 10"^. 



