NORMAL PROCESSES OF ENERGY METABOLISM 531 



read. By absorption of oxygen from the animal chamber the oil-drop is 

 shifted toward that chamber and whenever a reading is taken a drop is 

 brought back to the zero mark by means of the pressure screw on the mer- 

 cury column. The volume of mercury moved upward then is equal to the 

 volume of oxygen absorbed when corrected from the original temperature 

 and barometric pressure to and 760 mm. The carbon dioxid is absorbed 

 as rapidly as produced by a drop of caustic soda placed in the bottom of the 

 animal chamber, the animal of course being protected from contact with 

 the solution. The production of carbon dioxid can be determined if, in 

 a control period, a small amount of water is used instead of the alkali. 

 The pressure change will then indicate the difference between the volume 

 of oxygen absorbed and the carbon dioxid given off. If the oxygen absorp- 

 tion is determined just before and just after this under conditions other- 

 wise the same, the volume of carbon dioxid will be found by substracting 

 the difference-volume from the volume of oxygen. The respiratory quotient 

 is then available. 



It is obviously necessary to keep the two chambers in the same water or 

 oil bath in which the liquid is sufficiently stirred so that the two chambers 

 shall be of exactly the same temperature. 



The micro-respiration apparatus of Krogh follows very similar prin- 

 ciples. With it Krogh was able to follow the oxygen absorption of a single 

 insect egg weighing about 2 mgm. in ten-hour periods from immediately 

 after it was laid until the hatching of the larva ( Krogh (&)). 



II. Methods for Measuring the Respiratory Exchange 

 by Direct Connection with the Repiratory Passages 



The first observations upon the respiratory exchange of man made by 

 Lavoisier provided for the direct examination of the expired air. A copper 

 mask was used fitting tightly over the subject's face and by some means 

 not clearly understood the inspired air was separated from the expired air, 

 which was passed into alkali, thereby removing the carbon dioxid. Many 

 different modifications of the original method of Lavoisier have been de- 

 vised. Those which employ means to separate the inspired air from the 

 expired air and provide for the collection or automatic analysis of the latter 

 should be described under the rubric of "open circuit" or air-current types 

 of apparatus. Other methods employ some form of "closed circuit" ap- 

 paratus. 



1. Open Circuit Instruments, a. Mouthpieces, Nose-pieces, Masks. 

 For connection of the apparatus to the respiratory passages of the subject 

 a rubber mouth-piece originally constructed by Denayrouse for the use of 

 divers has been widely employed. It consists of a wide rubber disc which 

 fits in between the lips and the teeth of the subject. In the middle of this 

 disc is a 2 cm. opening leading into a rubber tube of the same size. On 



