The aftergrowths obtained under the usual working conditions vary according to the dosage of chlorine employed, and none of the above hypotheses alone provides an adequate explanation. When the dosage is small, a small number of active organisms, in addition to the spore bearers, will escape destruction, and others will suffer a reduction of reproductive capacity. The flora of the aftergrowth in this case will only differ from the original flora by the elimination of a majority of the organisms that are most susceptible to the action of chlorine and the weaker members of other species of greater average resistance. As the dose is increased these factors become relatively less important until a stage is reached when only the most resistant cells, the spores, remain. The resultant aftergrowth must necessarily be almost entirely composed of spore-bearing organisms. A small number of the most resistant members of non-sporulating organisms may also be present but they will, in the majority of instances, form a very small minority. This is the condition that usually obtains in practice and it is necessary to consider whether the aftergrowth may have any sanitary significance. Concerning the secondary development of _B. coli_, the usual index of pollution, there is but little information. H. E. Jordon[6] reported that, of 201 samples, 21 gave a positive _B. coli_ reaction immediately after treatment, 39 after standing for twenty-four hours, and 42 after forty-eight hours. These increases were confined to the warm months, the cold months actually showing a decrease. The following figures, taken from the author's routine tests for 1913 and 1914, show a similar tendency, but an analysis of the results by months did not show that this was confined to the warm season. The sequence of the results from left to right, in the following Table, is in the same order as the contact period. Approximately 290 samples were taken at each sampling point. At station No. 2 the germicidal action was still proceeding but at No. 5, representing an outlying section of the city, the increase in the _B. coli_ content is very apparent. During 1915 and 1916 the author endeavoured to duplicate these results under laboratory conditions and entirely failed. These experiments, which were made with the same materials as were in use at the city chlorination plant, but in glass containers, were usually only carried to a forty-eight hours contact, as this was the extreme limit for the city mains; one, however, was prolonged to five days. Many experiments were made under varying conditions, with similar results. Typical examples are given in Tables VI, VIII and IX on pages 33 and 37. TABLE XIX.--AFTERGROWTHS OF B. COLI PERCENTAGE OF SAMPLES SHOWING B. COLI IN 10 C.CMS.