336 



Dissolved Oxygen in Rain-water 



These results show (1) that with normal clean rain there is no 

 absorption of oxygen in 24 hours ; the small difference at 25° is almost 

 certainly due to the 42 per cent, supersaturation ; (2) that the paraffin 

 has no more action than the glass stopper; (3) that rain-water, like 

 tap- water, if it is not shaken, can become strongly supersaturated 

 without loss of oxygen. 



Fig. 1 shows the dissolved oxygen in the rain-water samples arranged 

 in the order of collection during the year, compared with Dittmar's 

 figures for distilled water saturated with air at the temperatures of the 

 rain-water when collected. 



Dissolved oxygen. 



Parts per 100,000 



by weight 



1-4 

 1-3 

 1-2 



M 



1-0 

 0-9 

 0-8 



0-8 



80 



90 



120 



19-0' Q" 



Dissolved oxygfn in distilled water saturated with air at temperature of 



rain-water when (•olle;'ted. 

 Dissolved oxygen in rain-water. 



Fis. 2. 



In Fig. 2 the samples are arranged in order of temperature when 

 collected. It will be noticed that the first four samples, collected at 

 temperatures below 5° C, are considerably under saturation. In these 

 cases the rain as it fell was at a temperature perhaps 5° above that which 

 it assumed after standing some hours at a much lower air temperature. 

 The interval between the rain falhng and the collection of the sample 

 was long enough to lower the temperature of the rain-water in the 

 bottle but not long enough for the exposed sample to saturate itself 



