50 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



used up will have no influence whatever on the respiration or gas ex- 

 change, and when the oxygen content of the water does not sink below 

 4 to 5 c.c. per litre the exchange will in almost all cases remain per- 

 fectly normal. The principle of this method is very old, since it was 

 employed in the researches of Humboldt and Provencal in 1809, but 

 the technique has, of course, been improved repeatedly. 



It is important to guard against the action of bacteria by limiting 

 the duration of the experiment to twelve hours or less (except at very 

 low temperatures), and it must further be borne in mind that when an 

 animal, and especially a fish, is transferred from one vessel to another 

 a considerable time will often elapse before it becomes quiet and the 

 respiratory exchange normal (Lipschiitz [1911]). 



The technique is extremely simple. A bottle is selected which will 

 hold so much water that the animal can use up about one-third of the 

 oxygen in a suitable time (generally one to six hours). The volume of 

 the bottle is measured and it is filled with water. It is generally desir- 

 able to shake the water with air at a temperature just above that at 

 which the experiment is to be performed because that prevents the 

 liberation of air bubbles during the experiment. The animal is put in, 

 and when it has become quiet some more water is run through the bottle 

 and a sample taken for analysis. The bottle is closed and the tem- 

 perature kept constant during the experimental period. When the 

 animal is sluggish it is necessary to mix the water gently before taking 

 samples. 



A number of more or less complicated respiration apparatus for 

 aquatic animals have been constructed by Jolyet and Regnard [1877], 

 Grehant [1886], Zuntz [1901] and Bounhiol [1905]. They are founded 

 on the Regnault principle, and consist of a water jar introduced in a 

 closed-space apparatus with air which circulates so as to aerate the 

 water in the jar. Analyses of gases both in the water and in the air 

 are necessary when such an arrangement is adopted, and no material 

 advantages are gained to compensate for the complications and the loss 

 of time involved. The only advantage claimed is that the quantity of 

 dissolved oxygen can be maintained nearly constant over a long period 

 and in a comparatively small volume of water, which factors will make 

 for increased accuracy ; but since an experiment cannot be extended 

 over a long period on account of the bacteria, and since a decrease of 

 3 c.c. of oxygen per litre, which can be measured with an accuracy of 

 about I per cent., does not afifect the respiratory exchange, the ad- 

 vantage is apparent only. 



