These results demonstrate in a most striking manner the beneficial effect of chlorination. The general conditions, with the exception of the raw water supply, were approximately the same in all four districts. Diagram XIII shows that the raw water supply of District No. 1 was slightly worse than any of the others, 21.8 per cent of the samples from District No. 1 containing _B. coli_ in 1 c.cm. as compared with 21.0 per cent in the most polluted supply of the other districts. [Illustration: DIAGRAM XIII B. COLI IN CHICAGO RAW WATER (Young)] The results obtained at Ottawa are also conclusive. Following two epidemics of typhoid fever in 1911 and 1912, caused by breaks in the intake pipe, hypochlorite treatment was commenced and has been in continuous operation until February, 1917, when chloramine treatment was substituted. The dosage has been so regulated as to assure a high degree of purity at all times in the water delivered to the mains and as evidence of this it might be mentioned that the average _B. coli_ index (calculated by Phelps' method) for the years 1916 and 1917 was only 0.27 per 100 c.cms. The typhoid rates for the five years preceding the epidemic years and for a similar subsequent period are given in Diagram XIV. [Illustration: DIAGRAM XIV TYPHOID IN OTTAWA] The diagram shows that there has been a constant reduction in the city typhoid rate since the last severe epidemic with the exception of the year 1915. The high rate of that year was caused by a localised epidemic started by polluted well water and spread by flies from an unsewered area. This outbreak was the cause of about seven deaths registered during that year (population 100,000). The objection might be raised that if the reduction of the typhoid rate were due to the water treatment, the decline should have been abrupt and not a gradual one. It is probable that there has been practically no water-borne typhoid in the city since chlorination was commenced but this fact is masked by cases from other sources. During 1911 and 1912 over 3,500 cases of typhoid were reported, of which an appreciable number would become carriers for various periods of time. As these carriers decreased the number of cases infected by them would also decrease and so account for a gradually declining death rate. It might be further objected that the reduced typhoid rate is due to a general improvement in the sanitary conditions. If the death rate from causes other than typhoid can be regarded as a measure of the general sanitary conditions it is obvious from the data in Table XXXIII that the improvement in the typhoid rate is immeasurably greater than can be ascribed to that cause. TABLE XXXIII.--DEATH RATES IN OTTAWA BEFORE AND AFTER CHLORINATION