DISSOLVED OXYGEN 347 
from that point as outlined, except that only 1 ¢.c. of alkaline potas- 
sium iodide need be added. 
Calculation of Results. Oxygen shall be reported in parts per million 
by weight. It is sometimes convenient to know the number of cubic 
centimeters per liter of the gas at 0° C. temperature and 760 mm. 
pressure and also to know the percentage which the amount of gas 
present is of the maximum amount capable of being dissolved by dis- 
tilled water at the same temperature and pressure. If 200 c.c of the 
sample is taken the number of cubic centimeters of N/40 thiosulphate 
used is equal to parts per million of oxygen. Corrections for volume of 
reagents added amount to less than 3 per cent and are not justified except 
in work of unusual precision. To obtain the result in cubic centimeters 
per liter, multiply the number of cubic centimeters of thiosulphate used 
by 0.698. To obtain the result in percentage of saturation consult 
the table on page 68 of the 1917 edition of Standard Methods for the 
Examination of Water and Sewage, and divide the number of cubic 
centimeters of thiosulphate used by the figure in this table opposite 
the temperature of the water and under the proper chlorine figure. 
The last column of this table permits interpolation for intermediate 
chlorine values. At elevations differing considerably from mean sea 
level and for acctirate work, attention must be given to barometric 
pressure, the normal pressure in the region being preferable to the 
specific pressure at the time of sampling. The term “ saturation ”’ 
refers to a condition of equilibrium between the solution and an 
oxygen pressure in the atmosphere corresponding to 158.8 mm., or 
approximately one-fifth atmosphere. The true saturation or equilib- 
rium between the solution and pure oxygen is nearly five times this 
value and consequently values in excess of 100 per cent saturation fre- 
quently occur in the presence of oxygen-forming plants. 
When the oxygen concentration becomes too low, the stream becomes 
putrescible. This same condition may be duplicated in a bottle by 
allowing it to stand full of sewage tightly stoppered. The following 
things will be noticed: formation of evident amounts of hydrogen 
sulphid; heavy deposits of black sediment; disappearance of dissolved 
oxygen. These are conditions which are not desired in streams and 
they indicate that the available oxygen is insufficient to care for the 
organic matter. 
Stability and Relative Stability. A stream is stable when its oxygen 
demand has been satisfied; the products resulting from the decomposi- 
tion of the organic matter should be completely oxidized. Phelps has 
stated that this condition obtains when the available oxygen exceeds the 
