THE DETECTION OF SEWAGE CONTAMINATION. 245 
amount entirely consumed in the life processes of the animals is 
also great. 
As named above, I kept cyclops alive for many months by feed- 
ing them on human excrement. It is thus easy to understand 
why, when they abound in the Thames, the relative amount of 
human excrement is very considerably less than in the winter, 
when their number must be much smaller. 
We thus appear to be led to the conclusion that when the 
amount of sewage discharged into a river is not too great, it 
furnishes food for a vast number of animals, which perform a 
most important part in removing it. On the contrary, if the 
discharge be too great it may be injurious to them, and this pro- 
cess of purification may cease. Possibly this explains why in 
certain cases a river, which is usually unobjectionable, may occa- 
sionally become offensive. It also seems to make it clear that the 
discharge of rather too much sewage may produce relatively very 
great and objectionable results. 
Though such comparatively large animals as entomostraca may 
_remove much putrefiable matter from a river, we cannot suppose 
that, except incidentally, they remove such very minute objects as 
disease germs, but it would be a subject well worthy of investiga- 
tion to ascertain whether the more minute infusoria can, and do 
consume such germs as a portion of their food. If so, we should 
be able to understand how living bodies, which could resist any 
purely chemical action likely to be met with in a river, could be 
destroyed by the process of minute animals. Hitherto I have had 
no opportunity for examining this question critically, but have 
been able to learn certain facts which, at all events, show that it is 
well worthy of further examination. It is only during the last 
month that I have paid special attention to the number of the 
larger infusoria, and various other animals of similar type, met with 
per gallon in the water of rivers and the sea, which can be seen 
and counted by means of a low magnifying power. At low water 
in the Medway above Chatham, in the first half of June, the 
average number per gallon has been about 7,000, but sometimes 
as many as 16,000. Their average size was about zgoth inch. 
Possibly the number of still more minute forms may be equally 
great; but, even if we confine our attention to those observed, we 
cannot but conclude that their effect in removing organic matter 
must be very considerable ; and judging from what occurs in the 
case of larger animals, those ;,/5,th of an inch in diameter may 
well be supposed to consume as food, particles of the size of 
germs. Up to the present time I have, however, collected so few 
facts bearing on this question that it must be regarded merely as 
a suggestion for future inquiry. 
So far, I have referred exclusively to the effect of animal life. 
