RESPIRATORY METABOLISM 357 



excellent treatise of Dixon (1934) is recommended. In this publication 

 the theory and the more common forms of the apparatus are described 

 in detail. 



The Barcroft differential type can be made sufficiently small and 

 sensitive for the study of respiration of Protozoa, when moderately con- 

 centrated mass cultures are available. A manometer designed by Dr. 

 T. C. Evans (modified from that of Bodine and Orr, 1925), which 

 has been in use in this laboratory for some time, seems to be quite 

 suitable. It resembles the standard Barcroft (Dixon, 1934, p. 37) ex- 

 cept that the two stopcocks are replaced by one double stopcock, which 

 insures the simultaneous opening and closing of both flasks. The cups 

 may be relatively small (about 5 cc), and the U-shaped portion of 

 the capillary (0.3 mm. bore) is placed at an angle of about twenty 

 degrees from the horizontal. This design combines a high degree of 

 sensitivity with ruggedness and dependability. The Duryee ( 1936) modi- 

 fication of the Thunberg-Winterstein principle and the Fenn (1928) 

 form of microdifferential respirometer also seem to have a sensitivity 

 adequate for moderately concentrated suspensions. Schmitt (1933) has 

 devised an extremely sensitive form of microdifferential manometer, in 

 which the gain in sensitivity and accuracy is due chiefly to an elaborate 

 reading device and a system of temperature control which is stated to 

 make meniscus movements of as little as one micron both detectable and 

 significant. 



4. CAPILLARY MANOMETER 



The use of capillary tubes for measuring respiration of single protozoan 

 cells was introduced by Kalmus (1927, 1928a). This method was 

 improved by Howland and Bernstein (1931), who by means of a micro- 

 injection device drew small amounts of oil, air, KOH solution, and 

 water containing an animal into small capillary tubes, so that they were 

 finally arranged in the following order: oil, KOH, air, animal in water, 

 oil. As the animal consumed O^, the distance between the oil-water and 

 the oil-KOH interfaces decreased. This change was measured micro- 

 scopically by means of an ocular micrometer and a calibrated mechanical 

 stage. By using control tubes made in a similar manner but without an 

 animal, it was possible to correct for slight movements due to thermo- 

 barometric changes and to osmotic differences between the water and 



