230 BEVIEW— TROPICAL MEDICINE, ETC. 



Water— should be cleaned at regular periods by removing a thin layer of sand half an inch in thickness 

 continual and disturbing the upper part of the remainder of the sand by means of a fork or rake, 

 so as to expose it to the air. This process is repeated till the upper layer of fine sand is 

 reduced to a foot in thickness, after which the whole filter is cleansed. Koch maintains 

 that for efllcicnt filtration the sand should not be below a foot in thickness, the rate of 

 flow should not be more than 3-95 inches per hour, and the number of microbes should 

 not exceed 100 per c.c. in the filtered water. 



There are two systems of sand filtration — the slow and the rapid. In the former, the 

 process is dependent on the gravitation of the water through the filter ; in the latter, by the 

 aid of mechanical contrivances, the rate of filtration is generally about fifty times as great 

 as that of the ordinary gravity filter. In these mechanical filters, an artificial film of an 

 intercepting nature is obtained by the addition of some substance, usually sulphate of 

 alumina, and they are, as a rule, in covered vats or tanks and subjected to pressure by the 

 admission of compressed air. For domestic purposes the Pasteur-Chamberland and 

 Doulton filters are useful, as they render the water bacteria free. Asbestos, charcoal and 

 similar filters now possess only an historical interest as they are quite inefficient. The 

 utility of Pasteur-Chamberland filters is greatly inhibited in the Tropics owing to the 

 frequent muddy condition of the water, which completely blocks up the filter, hence 

 necessitating frequent cleansing. This may be overcome to a certain extent by clarifying 

 the water prior to filtration. 



In the Union Pharmaceutique a simple and apparently harmless method is described for 

 purifying potable water.' The method, as described by M. Celestion Hy, is based on 

 that originally devised by MM. Gerard and Bordes. Tlie water to be purified is first treated 

 with a powder consisting of one part potassium permanganate to seven parts sodium 

 carbonate and slaked lime. After five minutes interval, eight parts of anhydrous ferrous 

 sulphate are added. The method is based upon the fact that potassium permanganate in an 

 alkaline solution oxidises organic matter and destroys micro-organisms. The sodium 

 carbonate precipitates any calcium sulphate that may occur naturally in the water, and the 

 calcium hydrate precipitates any bicarbonate of lime that maj' be present. On adding 

 ferrous sulphate, the excess of permanganate is removed in the form of a dense precipitate. 

 The water drawn off from the precipitate is pure and limpid and contains only a small 

 amount of the sulphate of potassium and sodium. Their presence is not at all objectionable 

 and there is very little danger in using an excess of either of the powders. Howard Jones, - 

 in an interesting paper, describes a method of efficiently removing from reservoirs the 

 objectionable smell caused by the presence of algse in the water supply for Newport. 

 Having noted the results obtained in America by the use of copper sulphate for the removal 

 of algae, it was decided to employ copper sulphate. Ten pounds of copper sulphate were 

 used for every million gallons of water, the copper sulphate being towed in bags behind a 

 raft. This treatment was successful in the course of a few days. 



With regard to the value of storage as a method of purifying water, Houston^ 

 maintains that the chief importance of storage lies in the fact that the micro-organisms of 

 water-borne disease gradually die in the struggle for existence when they have to contend 

 against the ordinary water bacteria. If water is stored for weeks and months, the 

 probability of any harmful bacteria surviving is excessively remote, and, if stored 

 sufliciently long, is incapable of giving rise to epidemic disease. Accordingly the 

 Metropolitan Water Board of London opened two new reservoirs for the storage of water, 

 which would therefore pass through a sedimentary stage before reaching the filter beds. 

 Eeference may be made to the presence of Crenothrix in a sample of water which was 

 taken from some wells sunk in the vicinity of Khartoum. A note on Crenothrix polyspura 

 (Cohn), is given by Rullmann,'' who found it in the reservoirs supplying water to the town of 

 Landshut, in lower Bavaria. He obtained micro-photographs of this alga stained with 

 carbol-fuchsine. 



' (April 4tb, 1905), " The Purification of Potable Water." Lancet. 



- Howard Jones (January Ist, 1907), "Copper Sulphate Treatment of Reservoirs." Public Health, 

 p. 244, Vol. XIX., No. 4. 



■■' Houston, A. C. (June, 1907), " Sedimentation in the Purification of Water." Public Health, p. 558, Vol. XIX., 

 No. 9. 



* Rullmann, W. (December, 1907), Photogramme von Crenothrix Polyspora, Cohn. Ccnt.filr. Bakt., II. Ab., 

 Bd. XX., No. 4/5. 



