before and. after recewing the Cambridge sewage effluent. 357 
bacteria, resembling closely the anthracoid group. Each rod 
was attached to the next, and no free members were seen; they 
had the appearance of spore formation in the centre of each 
rod; they were more abundant in the effluent in cold weather. 
It is proposed to study them in more detail later. (b) The 
amount of oxygen absorbed by the river above the outfall was 
always above the standard of a clean river water (0:2 gram) in the 
winter months from October to December inclusive, whereas in 
the summer months, from May to August inclusive, it was 
always below the standard. It cannot be said, therefore, that 
the river itself always satisfies the standard of the Royal Com- 
mission. (c) Only on two occasions (August 14 and October 10) 
was the amount of oxygen taken up by the effluent below the 
standard of the Commissioners. (d) On the other hand, the 
purification which takes place when the effluent mixes with 
the river water is fairly rapid; and, although the mixture of 
the effluent and river at 50 feet below the outfall gives a figure 
which is sometimes above and sometimes below the standard of 
0-4 gram of oxygen absorbed in five days at 18°C., yet the 
river + mile down as regards its cleanliness compares fairly well 
with the standards of a diluting water suggested by the Com- 
missioners. For example, five of the analyses would grade the 
river as “clean,” ten as “fairly clean” and three as “doubtful ” 
at + mile below the effluent outfall. 
Three factors at least may be suggested to explain the 
difference in -the results of the summer and winter months. 
They are (1) the rainfall, (2) the number of hours of sunshine 
and their influence upon aquatic vegetation, and (3) the tem- 
perature. The curves in Fig. 1 show that as the rainfall increases, 
the amount of oxygen absorbed in five days also increases; and, 
it will be noticed, that this takes place at the beginning of the 
rainy season in October. An increase in the rainfall is accom- 
panied by an increase in the pollution of the river, and a corre- 
sponding increase in the oxygen absorbed, and this is well shown 
by the two curves rising and falling together. The second factor 
is the presence of aquatic vegetation influenced by the number 
of hours of sunshine; and the curves of Fig. 2 illustrate the 
variation in the amount of dissolved oxygen which rises and falls 
with the number of hours of sunshine in the summer months. 
It is well known that aquatic plants give off more oxygen under 
the influence of the sun than in its absence, and this fact 
explains the increase in the dissolved oxygen. Such an ex- 
planation is confirmed by the curves of Fig. 3 for the winter 
months, where there is no regularity like that indicated by the 
curves in Fig. 2. Although there is a decrease in the number of 
hours of sunshine, and an almost entire absence of aquatic plants, 
