The removal of intestinal forms is, of course, merely a relative one and when large quantities of treated water are tested their presence can be detected.
The author, in 1915, made a number of experiments to ascertain whether the _B. coli_ found after chlorination were more resistant to chlorine than the original culture. The strains surviving treatment with comparatively large doses were fished into lactose broth and subjected to a second treatment, the process being repeated several times. The velocity of the germicidal reaction with the strains varied somewhat, but not always in the same direction, and the variations were not greater than were found in control experiments on the original culture. No evidence was obtained that the surviving strains were in any way more resistant to chlorine than the original strain; in considering the results it should be borne in mind that the surviving strains were cultivated twice on media free from chlorine before again being subjected to chlorination.
A number of the strains that survived several treatments were cultivated in lactose broth and the acidity determined quantitatively. All the cultures produced less acid than the original culture, and the average was materially less than the original. These results point to a diminution of the bio-chemical activity by action of the chlorine.
A point of perhaps more scientific interest than practical utility is the relative proportion of the various types of _B. coli_ found before and after treatment with chlorine. The author, in 1914, commenced the differentiation of the types by means of dulcite and saccharose and obtained the results shown in Table XVIII. These figures are calculated from several hundreds of strains.
Although there is a slight difference in the relative proportions of the types found at Ottawa and Baltimore, both sets of results show definitely that there is no difference in the resistance of the various types to chlorination.
=Aftergrowths.= In Tables XIII (p. 44) and XV (p. 51), it will be noticed that, after the preliminary germicidal action has subsided, a second phase occurs in which there is a rapid growth of organisms. This is usually known as aftergrowth. When the contact period between chlorination and consumption is short, the reaction does not proceed beyond the first phase, but when the treated water is stored in service reservoirs the second phase may ensue. At one purification plant, where the service reservoirs are of large capacity, the aftergrowths amounted to 20,000 bacteria per c.cm. although the water left the purification plant with a bacterial count usually lower than 50 per c.cm.