Apvrnry— Studies in Chemical Analysis of Fresh & Salt Waters. 361 
Although it is well known that accurate determinations of the 
actual quantity of oxygen dissolved in the water may be made, 
within certain limiting conditions, by Schiitzenberger’s, or by 
Thresh’s, volumetric method, a serious difficulty arises when it 
comes to the question of the standard of oxygen saturation to 
be taken for the purpose of calculation. This becomes evident 
immediately it is remembered that the composition of the dissolved 
gases in still water alters very slowly with alterations of tempera- 
ture, so that samples of a good fresh river water may be, and 
probably always are, either over-saturated or under-saturated with 
oxygen for the temperatures of the water at the times of collection. 
When it happens that the waters examined are pure sea water, 
or mixtures of sea and river waters, such as in tidal rivers, the 
error becomes still more serious, since the solubility of atmospheric 
oxygen is very much less in sea water than in fresh, temperature for 
temperature. 
If, however, the dissolved nitrogen, as well as the oxygen, be 
determined, we have thereby the means of calculating whether the 
volume of dissolved oxygen found does actually represent the state 
of oxygen saturation for the conditions to which the water had 
been subjected at the time it was collected, or of calculating, if it 
does not, its exact amount below that state. 
The truth of these statements will be apparent from the follow- 
ing table, showing the solubility of atmospheric nitrogen and 
oxygen at different temperatures in distilled and in sea waters :— 
The gases are expressed in c.cs., at N.T’.P., per 1000 c.cs. of 
water. 
Sea-water.? Distilled water.? 
Temperature. Sea-water. Distilled water. 
Nitrogen.| Oxygen. | Nitrogen.] Oxygen. 
N No 
PO, 12-4 6°45 15°47 7°87 O02. = —— ~ 
we d 1-933 1-966 
Ne N 
19? Ce 11°34 5°83 13°83 7:09 On = 
1:945 1:951 
Ne Ne 
0° C. “41 ‘31 12°76 6°53 ,= = = 
Z EY : : Ye 1:960 1°954 
z No No 
HC. 9-62 4°8 11°81 5°97 2,= 
: O2 1-975 1-978 
1 Dittmar, “‘Challenger”’ Reports, p. 224. 
2 Roscoe and Lunt, Journ. Chem. Soc., 1889, 552. 
