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. TABLE XVIII.--TYPES OF _B. COLI_ SURVIVING CHLORINATION ------------------+------------------------------------------------ | PERCENTAGE OF ORGANISMS. +-----------+-----------+-----------+------------ | B. coli | B. coli | B. lactis | B. acidi | communis | communior | aerogenes | lactici +-----+-----+-----+-----+-----+-----+-----+------ | |Chlo-| |Chlo-| |Chlo-| |Chlo- |Raw. | ri- |Raw. | ri- |Raw. | ri- |Raw. | ri- | |nated| |nated| |nated| |nated ------------------+-----+-----+-----+-----+-----+-----+-----+------ Ottawa, 1914 | 5 | 4 | 40 | 48 | 44 | 36 | 11 | 12 Ottawa, 1915 | 8 | 8 | 50 | 46 | 34 | 31 | 8 | 15 Baltimore, 1913[A]| 11 | 14 | 33 | 25 | 35 | 31 | 21 | 30 ------------------+-----+-----+-----+-----+-----+-----+-----+------ [A] Thomas and Sandman.[5] Regarding the nature of this aftergrowth, there has been a considerable difference of opinion: some regard it as the result of the multiplication of a resistant minority of practically all the species of organisms present in the untreated water; others, that it is partially due to the organisms being merely "slugged" or "doped," i.e. are in a state of suspended animation, and afterwards resume their anabolic functions; whilst others believe that with the correct dosage of chlorine, only spore-forming organisms escape destruction and that the aftergrowth is the result of these cells again becoming vegetative.