The experiments with individual strains in sterile water show a marked difference between the behaviour of the streptococci and B. coli. In all cases the streptococci diminished in numbers and in two experiments were practically eliminated at the end of 2 weeks while in the other two they did not survive very much longer. On the other hand the B. coli strains although in a medium which contained little or no organic matter, and which was not in any sense a nutrient medium, showed in every case multiplication, in most extensive multiplication. This was rather contrary to our anticipations as although we were prepared for prolonged survival under conditions of freedom from com peting bacteria we did not anticipate such marked increase in numbers. In Exp. XIX the bacillus tested was still present over fifty-fold of the original number at the end of 11 weeks.

We have given some attention to the characters of the surviving types of B. coli in the tank experiments. Generally speaking they were quite normal in respect of fermentation reactions but on three occasions, Exps. XIII, XV and XX, the capsulated “high ratio” (CO2: H) coli of Clark and Lubbs were found, and in Exps. IV, XX and XXI the surviving organisms isolated were also capsulated but not “high ratio” organisms. It appears possible that the capacity to form capsules has something to do with the viability of these organisms and Exps. XXII and XXIV are evidence in this direction, but it is to be noted that the surviving B. coli in Exp. XVI after 7 weeks produced no definite capsule when grown in milk.

In Exp. XX the original sewage was plated out upon lactose neutral red bile salt agar and 10 strains of B. coli isolated of which 50 per cent, showed capsule formation. Fourteen days later 11 strains were isolated from the sewage tank water and of these 90 per cent, were weak lactose fermenters and showed no capsule formation. In the same experiment 2 strains isolated after 4 weeks and another after 5 weeks showed definite capsule formation. The ultimate B. coli survivors were a pparently all capsulated organisms, although at the start as many non-capsulated as capsula ted organisms were present.

The tank experiments with excreta or sewage added to a large bulk of water yielded minor differences in the individual experiments, but in general they all show a rapid diminution and elimination of the streptococci and a continuous but not quite so rapid diminution inthe number of B. coli. With the latter it was more common to find persistence in small numbers for a period extending to many weeks. The elimination of the streptococci was particularly uniform and rapid. At the end of 2 weeks in only one experiment were they present in more than insignificant numbers.

While the diminutioji was rather more rapid for excreta than for sewage for both streptococci and B. coli, no definite constant differences in relation to the kind of contamination could be made out.

The decline curves of both organisms agree very closely as can be most readily seen if the figures are plotted out as graphs. Three of the experiments, in which the initial numbers of B. coli and streptococci were identical, and also Exp. XI are set out in this way.

The available data is hardly extensive enough to enable deductions to be drawn other than broad and general ones, but the facts add confirmation to the view that the presence of either streptococci or B. coli in considerable numbers, i.e. in 1 or even 10 c.c. of a water, can only indicate contamination considerable in amount and of recent origin.

In particular the finding of streptococci in any numbers can be accepted as indicating considerable and recent contamination. We consider that the streptococcus determination is very valuable on its positive side as an indication of recent contamination. As a means of judging of the recency of the contamination it is even more valuable than the B. coli enumeration.

Put another way our experimental data shows that in nearly every case there is a marked diminution in the number of both B. coli and streptococci at the end of even 1 week, so that it.follows that when these organisms are found in large numbers the contamination must have been either very recent or especially abundant.