: 
360 Messrs Purvis and Black, Oxygen content of the river Cam 
767 mm., on June 13, for example, 10086 c.c. of dissolved oxygen 
in the Cam water above the effluent outfall. On reference to 
Fig. 2 this corresponds to the maximum amount of sunshine for | 
the two preceding days. | 
To sum up this investigation as it affects the disposal of the 
sewage effluent into the Cam, it is evident that (1) the seasonal | 
variations had an important influence on the composition of the 
river as the receiver of the effluent, for the oxygen absorbed | 
figure was below the standard in the summer months and above | 
the standard in the winter months; (2) the suspended solids in | 
the effluent were above the standard on several occasions, par- 
ticularly in the summer months, when they were fecal solids and 
not masses of bacteria; (8) the oxygen absorbed in five days by 
the effluent satisfied the standard of the Commissioners only 
twice in eighteen times; (4) with the increased pollution of the 
river as the diluting medium during the winter months, although 
the sewage effluent was of much better quality than that dis- 
charged into the stream during the summer months, the oxygen 
absorbed figures of the mixture of the polluting discharge and 
the river 50 feet below the effluent outfall exceeded the standard — 
oxygen absorbed figure of 04, at the rate of 89°2 per cent. of the 
samples taken from October to December, as compared with 
25 per cent. of those collected from May to August; (5) on the 
other hand, there is the important fact that the recovery or self- 
purification of the river is fairly rapid as shown by the figures — 
for the oxygen absorbed at 4 mile down the river below the 
effluent outfall. Such purification is brought about partly by 
the rapid absorption of the dissolved oxygen from the air and 
which is being continually replenished, partly by the oxygen 
given off by aquatic plants under the influence of sunlight, and 
to some extent by the nitrates, when present in the effluent, 
which are produced from the oxidation of the sewage as it passes 
through the filter beds; and these influences come into action as 
oxidisers of the dissolved organic matters. It «should also be 
remembered that the river receives no further sewage pollution 
till it reaches Clayhithe, 1¢ miles below the effluent, where 
there is some contamination; but after that there is no 
pollution till the river reaches Stretham, 8 miles down. 
However, accepting the effluents as fairly representative, 
and without considering the rapid purification when they are 
mixed with the river, an increase in the volume of the river so 
that the ratio of the effluent and the river should be about 1 to 
20 in the summer, and about 1 to 25 in the winter, it is probable 
that the effluent discharged into the stream would satisfy the 
standards of the Royal Commission. It will be seen from the 
tables that the dilution of the effluent varied between 1 of 
