Clark and De Gage[2] found that the use of smaller amounts of coagulant during the period of combined disinfection and coagulation resulted in an increase of nearly 25 per cent in the quantity of water passed through the filter between washings, and also in a material reduction of the cost of chemicals, which averaged $2.62 per million gallons for combined disinfection and coagulation as against $4.86 for coagulation alone. The water used in these experiments was obtained from the Merrimac River at Lawrence. The effect of hypochlorite on the reduction of algæ growths on slow sand filters was first noticed by Houston during the treatment of the Lincoln supply in 1905. Two open service reservoirs were fed with treated water and were themselves dosed from time to time. "Previous to 1905 they developed seasonally most abundant growths, but during the hypochlorite treatment it was noticed that they remained bright, clear, and remarkably free from growths" (Houston[3]). Ellms,[4] of Cincinnati, has also noted the effect of hypochlorite on algæ. When the bleach was added to the coagulated water the destruction of the plankton was not as satisfactory as had been anticipated and it was found that large doses destroyed the coating of the sand particles and rendered the filters less efficient. The use of bleach in the filtered water basin was more successful and cleared it of troublesome growths. In 1916, during the treatment of the London Supply with bleach (dosage 0.5 p.p.m. of available chlorine), Houston made further observations on this point. The Thames water, taken at Staines, had previously been stored for considerable periods in reservoirs, but this necessitated lifting the water by pumps which consumed large quantities of coal that were urgently needed for national purposes. As a war measure, the storage was eliminated and the water treated with hypochlorite at Staines and allowed to flow by gravitation to the various works where the slow sand filters are situated. The treatment resulted in a marked reduction in the growths of algæ, the reduction in the area of filters cleaned in 1916 (June to September) as compared with 1915 being as follows: Percentage Filter Works. Reduction (Approximate). Grand Junction (Hampton) 6 Grand Junction (Kew) 43 East London (Sunbury) 30 Kempton Park 33 West Middlesex 56 A portion of this reduction can probably be attributed to the elimination of storage.