E. H. Richards 333 



and the author in the course of an investigation for the Royal Commis- 

 sion on Sewage Disposal^. 



The results of a year's observations at Rothamsted are set out in 

 Table II. It will be noticed that, taking Dittmar's^ figures for distilled 

 water saturated with air as a standard, the samples of rain-water col- 

 lected at temperatures below 15° C. are practically all only shghtly 

 under saturation (average 93 per cent.), but the samples taken in the 

 summer months are all less aerated (average 85 per cent.). The summer 

 samples collected by Reichard are also all considerably below saturation. 



The dissolved oxygen content of the samples collected under paraffin 

 was usually very shghtly below that of the samples exposed to air. 

 This difference may be due either to the taking up of oxygen from the 

 air by the exposed sample while it accumulated in the collecting bottle, 

 supposing it to be below saturation when it fell, or to absorption of 

 dissolved oxygen by biological oxidation of organic matter brought 

 down by the rain in the sample collected under paraffin. Possibly 

 both actions w^ere sometimes responsible for the difference observed. 

 There is no doubt that the second process does occur quite rapidly in 

 the case of samples collected from the first rain after a period of drought. 

 On August 14, 1916, after three weeks without rain the sample exposed 

 to air gave 0-62 parts per 100,000 and the paraffin insulated sample 

 0*61 at 17-5° C. or only 63 per cent, saturation. This rain was visibly 

 dirty and there is no doubt that some absorption took place during the 

 twenty hours that elapsed between the fall of the first drops of rain 

 and the analysis of the sample. On the other hand samples collected 

 during heavy rain on August 3, 1915, and analysed at once showed only 

 88 per cent, saturation. There was no time for any absorption of 

 oxygen here even if the sample had been dirty which was not the 

 case as there had been frequent rains in the two weeks immediately 

 preceding. 



To see how much dissolved oxygen rain-water would take up in the 

 maximum time elapsing between collection and analysis, a sample from 

 a heavy rainfall in the early morning was siphoned into six bottles at 

 11 a.m. the same day. This experiment was designed to show (1) if 

 there was any absorption of oxygen in 24 hours at 14° and 25° C, 

 (2) if the paraffin was responsible for any of the deficiency found in 

 the samples collected under a layer of oil, and (3) if rain falling at night 

 at a relatively low air temperature and kept in the collecting bottles 



^ Royal Commission on Sewage Disposal, 1th Report, vol. ii, Appendix, p. Ill (1911). 

 ^ Challenger Reports, Physics and Chemistry, vol. i, p. 160. 



