REVIEW — TKOPICAL MEDICINE, ETC. 229 



When the soldier is some distance from transport he must be supplied with means of Water- 

 readily obtaining pure water, and the use of Vaillard's red, white and blue tablets or continued 

 syniodules is recommended. These consist respectively of — (1) Potassium iodide 

 together with sodium iodide (blue) ; (2) tartaric acid (red) ; (3) sodium hyposulphite 

 (white). To purify a litre (nearly two pints) of water, dissolve, simultaneously and com- 

 pletely, one blue and one red tablet in two or three tablespoonfuls of cold water. A yellow- 

 brown liquid is obtained. Add this to the litre of water to be purified. Shake and mix well. 

 After 10 minutes add one white tablet, and the yellow water, after shaking, becomes 

 colourless and drinkable. The action is due to the liberation of free iodine which, in a dose 

 of 25 mm. to the litre of water, kills with certainty, in 5 to 10 minutes, the B. typhosus, 

 B. coli and V. choleras. In the above operation 60 mm. of iodine are produced. If the 

 tablets were dissolved in the whole quantity of water to be sterilised, the amount produced 

 would be much less, hence it is essential to carry out the operation as above described. 



Tablet No. 3 (white), transforms the iodine into iodide, and the quantity of the latter 

 is so small that it exerts no injurious influence. These syniodules are prepared 

 by MM. Lepinois and Michel, 7, Rue la Feuillade, Paris ; and can be obtained from 

 A. Lewino, 6, Castle Street, Falcon Square, London, England. 



Evan's sterilising tablets, prepared according to Nesfield's' process, are also useful for 

 sterilising drinking water, and their action consists also in the free liberation of iodine and 

 its ultimate transformation. (Evan's sterilising tablets can be obtained from Evans, Sons, 

 Lescher and Webb, 60, Bartholomew Close, London, England.) 



The bactercidal action of Nesfield's tablets was investigated by Windsor,- who found 

 that the addition of one 2-grain tablet of mixed iodide and iodate of sodium, and one 

 similar tablet of citric acid to 4 gallons of water previously sterilised will kill in one 

 minute typhoid and cholera microbes when these are added and are present in numbers 

 exceeding 50,000 per c.c. 



Chloros,^ a commercial preparation of the hypochlorite of soda, was used extensively 

 for the sterilisation of the Lincoln water supply. Used in a strength of 1 in 50,000, it was 

 found to render the water free from objectionable micro-organisms, and at the same time 

 was devoid of harmful properties to the consumer. 



The action of Chloros depends on the liberation of oxygen, which in its nascent form 

 acts as an oxidising agent. For the water supply of large communities, purification by 

 filtration is extensively employed, but as this system is not used to any great extent in the 

 Tropics, it will only be necessary to refer to the more important points in relation to filtration 

 methods. The ordinary filters for public supplies consist of water-tight basins of varying 

 depths, with sides and floor built of cement. In the floor are channels for collecting the 

 filtered water. The filter normally is about 5 or 6 feet in depth, and is built up from the 

 bottom with stones or pebbles covered by a layer of coarse gravel on which is placed a 

 layer of coarse sand and finally a layer of fine sand. The water rests several feet deep on 

 the filter surface, and should not be allowed to flow through it at a greater rate than 4 inches 

 per hour. The purifying action of a sand filter depends on the slimy deposit which 

 occurs on its surface. This deposit is composed of finely-divided clay, with powerful 

 absorbent properties, and a gelatinous mass consisting of bacilli, streptococci, alg£e and other 

 organisms which have been intercepted. Immediately below this slimy deposit is a layer of 

 nitrifying organisms. The formation of this vital layer on the filter may take any time 

 from three to twenty-four hours, and during this period the water which passes through is not 

 free from impurities and should be allowed to run waste. It is essential to fill the filter-bed 

 from the top, and the water should be allowed to stand in it at a depth of 3 feet for at least 

 twenty-four hours. 



Sand filters require constant supervision, otherwise the effects of purifying the water 

 are nullified and the filters become a source of danger. Filters ought to be covered to 

 prevent the possibility of their becoming frozen in cold climates, and in hot climates to 

 prevent the water becoming too warm and favouring the growth of algse. When the slimy 

 layer becomes too thick for the water to pass through, it should be removed. The filter 



» Nestiold, V. B. (Ootober, 1905), "On the Sterilisation of Drinking- Water by the Liberation of Free 

 Iodine." Journal of Preventive Medicine. 



' Windsor, C. P. (AiiCTuat, 1906), "The Bactericidal Power of Nesfield's Method of Purifying Water, 

 Indian Medical QazcUc. 



' " The Disinfecting Value of Chlorine." Public Ueallh Engineer. 



