24 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



of a glass vessel, the size of which is selected according to the size of 

 animal (not exceeding I or 2 gr.) experimented on. The bottom of the 

 vessel is covered with a layer of 2 per cent. NaOH, which will absorb the 

 carbon dioxide produced by the animal. The vessel is connected with 

 one branch of a very sensitive manometer of narrow bore (^ mm.), 

 the other branch of which is connected with an exactly similar vessel 

 the compensating vessel l charged with the same volume of 2 per 

 cent, caustic soda but without any animal. When an experiment is in 

 progress the two vessels are placed in the same water-bath which is 

 kept well stirred, and the whole is shut off from the atmosphere. The 

 absorption of oxygen by the animal will then become accurately re- 

 corded by the manometer, and can be read off at intervals of suitable 

 length. 



When the two vessels are of equal or nearly equal volume a pres- 

 sure difference of I mm. corresponds to V/ + v where V is the gas 

 volume of the animal chamber ( - absorbing fluid and animal), p the 

 pressure of I mm. of the fluid in the manometer (kerosene) expressed 

 in atmospheres, and v the volume of I mm. of the manometric tube. 



V/> must be reduced to o by multiplication with - - in which t 



273 + t 



is the temperature of the water-bath ; v, which is very small compared 

 with V/>, is reduced once for all to o and 760 mm. from the ordinary 

 room temperature and the average barometric pressure. With an 

 animal chamber of 25 c.c. each mm. variation in the pressure read off 

 on the manometer will correspond approximately to 2 cub. mm. of 

 oxygen absorbed. 



With an instrument of this kind the oxygen absorption of a single 

 insect egg weighing about 2 mg. has been followed in lO-hour periods 

 from shortly after it was laid until the hatching of the larva. 



A very similar apparatus was constructed earlier by Winter- 

 stein [1912]. The manometer consists of a drop of kerosene in a 

 horizontal tube. This drop is set at o and the level of the mercury in 

 the U-tube to the left (fig. 4) which is graduated in cubic millimetres is 

 read. By the absorption of oxygen in the animal chamber the drop of 

 kerosene is caused to travel, and whenever a reading is to be taken it is 



1 The use of a compensating vessel was introduced into the gas analysis by Petterson 

 and adopted for respiration apparatus by Thunberg [1905] and Krogh [1906]. So long 

 as the temperature of the compensating vessel remains the same as that of the animal 

 chamber, the effects upon the manometer of all changes in barometric pressure or in the tem- 

 perature of the bath in which both vessels are immersed are automatically compensated and 

 have no influence upon the measurements, 



