36 FRESH-WATER BIOLOGY 



causes them to sink. The diffusion currents bring them up again 

 (Johnstone). This is no doubt a matter deserving investigation. 

 Turbidity is important largely through its relation to Ught. Most 

 aquatic animals will tolerate much sediment, at least under 

 experimental conditions. 



Chemical factors are not directly or clearly separable from 

 factors that may be regarded as physical or biological. Under 

 this heading are considered dissolved gases, inorganic salts, 

 acidity, alkaUnity, and neutrality. 



In order to support animals and plants continuously water must 

 contain certain minerals and gases in solution. Salts (carbonates, 

 sulphates, and chlorides) of magnesium, calcium, potassium, and 

 sodium, and salts of iron and siUcon are practically always in solu- 

 tion in water and their presence in definite proportions is beheved to 

 be essential to the Hfe of organisms. Pure distilled water has been 

 shown to be harmless to certain animals for comparatively short 

 periods but it is doubtful if it will sustain life indefinitely. Dis- 

 solved gases in definite proportions are essential. 



The occurrence of gases and their solubility under experimental 

 conditions are shown in Table VI. A standard method of express- 

 ing quantity of gas in solution is in cubic centimeters per liter at 

 0° C. and 760 mm. of mercury. Values are commonly given in 

 these terms. 



Nitrogen is the most inert and least important of the dissolved 

 gases. It rarely has any direct effect on animals and plants and 

 this apparently only when present in considerable excess of satu- 

 ration. Under such conditions it accumulates in the blood vessels 

 and tissues of fishes, crayfishes, insects, etc. In the organs of 

 circulation it may thus stop the blood flow and the animals die of 

 asphyxia. Birge and Juday state that in lakes in the region of 

 the thermocline and below an excess of 1 2 to 38 per cent of satura- 

 tion occurs, but under the conditions of pressure there this would 

 have no effect. It is probable that in nature this condition of 

 excess is not commonly great enough and does not often occur for 

 a time long enough to cause any fatal results. Several hours or 

 days, depending upon the excess, are required. Excess nitrogen is 

 a great source of difficulty in aquaria. 



