METHODS OF MEASURING RESPIRATORY EXCHANGE 25 



brought back to o by means of the screw. The volume of mercury added 

 is equal to the volume of oxygen absorbed, but this volume must, of 

 course, be reduced from the temperature and barometric pressure at 

 the moment of closing the apparatus to o and 760 mm. In Winter- 

 stein's apparatus it is not necessary to measure the volume of the ani- 

 mal chamber or the bore of the manometric tube. The calculation of 

 the results is, therefore, greatly simplified. The apparatus can be 

 made still more sensitive than Krogh's, but is not well adapted for pro- 

 longed experiments. A later model [1913] of Winterstein's apparatus 

 is shown in fig. 4. It is provided with four taps and connections, 

 which render possible the filling of the vessels with definite gas mix- 

 tures differing from the atmospheric. 



Both these instruments show only the oxygen absorption. The 

 production of carbon dioxide can be determined indirectly if the ani- 

 mal chamber is charged with a drop of water instead of the absorbing 

 soda lye. The pressure (or volume) change will then correspond to 

 the difference between the volume of oxygen absorbed and of carbon 

 dioxide liberated, and if the oxygen absorption is determined just be- 

 fore and just after such an experiment, the CO 2 production and the 

 respiratory quotient can be found by combining the results. 



Krogh's apparatus is a modification of Barcroft's blood gas appara- 

 tus. Winterstein's is an improvement on various earlier types (Thun- 

 berg [1905,2], Winterstein [1905, 1906], Widmark [1911]), which 

 were not suitable for accurate quantitative measurements. 



Thunberg's original micro-respirometer [1905, i] was a gas- 

 analysis apparatus of the Petterson type for the determination of very 

 small percentages of CO 2 , in which the animals to be experimented on 

 could be introduced into the gas-measuring pipette. The change in 

 volume of the enclosed air would indicate the difference between the 

 volume of oxygen absorbed and of carbon dioxide produced, and when 

 that had been read off the air could be carried over into the potash 

 pipette and the CO 2 absorbed. The change in volume after absorp- 

 tion would then give the carbon dioxide produced. This apparatus is 

 too costly and complicated for general use. The introduction of mer- 

 cury into the animal chamber is also to be deprecated, even if it has 

 not been directly demonstrated that mercury vapour is harmful to the 

 invertebrate animals in question. 



All the instruments so far mentioned can be used only for very 

 small animals up to I or 2 grm. weight. The surface of the CO 2 ab- 

 sorbing fluid which cannot be renewed must be comparatively large 



