OXYGEN SUPPLY OF CERTAIN ANIMALS 317 



APPENDIX 



DESCRIPTION OF APPARATUS AND METHODS EMPLOYED 



1. Method for determining the amount of dissolved oxygen 



The method for determining the amount of dissolved oxygen in the 

 various samples of water tested, was the same as that employed by 

 Birge and Juday in their work on the dissolved gases of inland lakes. 

 This is known as the 'Winkler method' and is described in detail in 

 their ''Inland Lakes of Wisconsin. The Dissolved Gases of the Water 

 and their Biological Significance ('11)." It depends primarily on a 

 change of manganous hydroxide to manganic hydroxide by taking up 

 the oxygen dissolved in the water. The manganic hydroxide reacts 

 with hydrochloric acid, and chlorine is liberated, which in turn reacts 

 with potassium iodide, liberating iodine. The iodine liberated is 

 directly proportional to the amount of dissolved oxygen which the 

 water contained. The iodine can be titrated against standardized 

 sodium thiosulphate, us'ing starch as an indicator. The results, in 

 terms of cubic centimeters of oxygen per liter, can be computed by use 

 of a given formula. 



This method is the simplest and most satisfactory of the various 

 methods for oxygen determination and was compared by Birge and 

 Juday with the boihng method, which they also used, and found to 

 check within the limits of error. 



2. Methods used in obtaining oxygen-free water from the lake bottom 



The apparatus employed in obtaining samples of water from the 

 stagnated area was borrowed from the Wisconsin Geological and Natural 

 History Survey. The pump has been described and figured by Juday 

 ('16). It consists of a hand pump with a hose attachment. A cali- 

 brated line was fastened to the end of the hose so that the intake could 

 be regulated to any desired depth. The exit tube from the pump was 

 connected with a short glass tube which could be inserted to the bottom 

 of the collecting bottle. By pumping a large amount of water through 

 the bottle and then gradually removing the tube, water was obtained 

 from any depth in the lake without coming in contact with the upper 

 water or with the air. The bottles used had an average capacity of 

 about 250 cc. and were provided with a sealing attachment which 

 insured a tight stopper in the mouth of the bottle. By filling the 

 bottle level full and inserting the stopper obliquely, thus forcing out 

 some of the water at the surface, a sample of water was obtained in 

 which there were no bubbles of air. Samples taken in this manner 

 and tested by the Winkler method for oxygen were used in determining 

 the extent of the stagnated area in Lake Mendota. 



By lowering the intake hose to the surface of the mud, water con- 

 taining mud could be pumped up. The mud was used after settling 



