CONDITIONS OF EXISTENCE 39 



low as, for example, in fishes insufficient oxygen acts on the respira- 

 tory center through the development of organic acid in the blood 

 due to incomplete oxidation, and causes the respiratory movements 

 to be increased. There is some evidence that respiratory activity is 

 increased through direct reflex action through the gills and opercles. 

 This increased respiratory activity supplies plenty of oxygen. 



Ammonia occurs in minimal quantities in natural waters but 

 may be present in some quantity in sewage or gas works wastes. 

 Ammonia like the other gases (CO, SO2, and C2H4) introduced into 

 streams by gas works is not only extremely poisonous, but fishes 

 do not turn back from it when they encounter it and are often 

 overcome without giving the avoiding reactions which protect fishes 

 from excesses of other substances normal to fish environments. 



Methane is a saturated hydrocarbon and has minor effects upon 

 organisms though it may be present in the hypolimnion of lakes in 

 considerable quantity. Traces of carbon monoxide occur also. 



Hydrogen sulphide is usually present in very small quantities in 

 the bottoms of lakes and sewage contaminated streams. It is 

 very abundant in salt lakes and arms of the sea. It results from 

 putrefactions and from the reduction of sulphates through the 

 action of the bacteria which prey upon organic sulphur (Lederer). 

 Though very poisonous it is not ordinarily present in sufficient 

 quantity to injure fishes (Shelford and Powers) though its absorp- 

 tion of oxygen ^ reduces the amount of this gas very materially. 



Carbon dioxide is the most important gas in fresh water. In 



small quantities it is essential rather than detrimental to aquatic 



' Samples of water without oxygen must be handled with utmost caution as an ap- 

 preciable amount of oxygen wUl be absorbed through the surface exposed by the nar- 

 row neck of a 250 cc. bottle in a few seconds. Biologists are very likely to attempt 

 great accuracy in putting up solutions and to exercise insufficient care in taking and ti- 

 trating samples. For ordinary work, in making up solutions it is sufficient to weigh to 

 one decimal place; chemicals must be carefully selected; especially, KI. The normal 

 solutions used will not be correct if made by an unskilled person; a correcting factor 

 must be used which may as well be 0.876 as 0.989. Skill in titrating and standardiz- 

 ing with solutions made by a chemist should be acquired. For methods see Birge and 

 Juday, and Sutton. Routine sanitary analyses include several items of unknown or 

 doubtful value to living organisms and do not include some of the most important 

 determinations such as acidity, alkalinity, hydrogen sulphide, and carbonaceous 

 materials that might be absorbed as food. Determinations are often not made at 

 once, and samples are commonly not collected from important animal habitats within 

 the body of w-ater. 



