340 GASOMETRIC-VOLUMETRIC METHODS 



18. Equilibrate the temperature and read the vol. (Fi) in divi- 

 sions as for oxygen (steps 13-14, pages 333-4). If liquid is in the 

 capillary, subtract the length of the liquid column from the total 

 reading. 



19. Fill the cup with water and draw three fourths of it into the 

 syringe. Return the bubble to the capillary with water beneath it. 



20. Fill the cup with 10% sodium hydroxide, with the cup point- 

 ing downward and the bubble expelled into the alkali, some of which 

 is drawn into the capillary as soon as the bubble is free. 



21. To complete the carbon dioxide absorption, rotate the instru- 

 ment a few times with the cup pointing slightly downward. Then 

 with the instrument held vertically, cup down, suck the gas bubble 

 back into the capillary. 



22. Again adjust the temperature and take a gas vol. reading 

 (¥2). 



23. Calculate the carbon dioxide content in volume per cent from 

 the equation: 



Carbon dioxide = (T^ — T'ol/dOO/b);' 



where h is the vol. of sample (step 5a I, / is the gas correction factor 

 (page 334), and i is an empirical factor correcting for reabsorption 

 and incomplete extraction. For measurements on blood, i = 1.015. 



3. Membrane Interferometer Volumetry 



A sensitive indicator of pressure change which is based on the 

 principle of the interferometer was developed by Tobias (1942). 

 While the instrument has not yet been employed in biological 

 experimentation, a brief description will be given nevertheless, since 

 its further development and exploitation may be stimulated by so 

 doing. The instrument responds to pressure changes of the order of 

 0.05 mm. water, or about 0.004 mm. mercury. It consists of a pol- 

 ished microscope slide on which an ordinary cover slip (0.1 mm. 

 thick), having a hole ground through it about 2 mm. in dian:ieter, 

 is fastened with heavy shellac. A film of collodion is placed over the 

 cover slip and then another cover slip with a hole drilled through it 

 is fitted on top so that the holes of both cover slips coincide. Shellac 

 is also used to fix the positions of the film and the top cover slip. 

 The assembly is shown in Figure 109. When placed on the stage of 

 a low-power microscope, and illuminated with monochromatic light. 



