54 TRANSACTIONS OF THE [NOV, 39, 



Oxygen required to discharge 



the peaty color, 3.30 



Nitric anhydride, 0.231 



Chlorine, 0.4U6 



Total hardness, 3.5 



Total solid residue at 110° C, 11,20 



Volatile matter at red heat, 10.25 



Fixed mineral matter, 0.95 



The oxygen gas dissolved in one litre was 5.17 cubic centi- 

 meti'es, the carbon dioxide 5.27 cubic centimetres, and the sum 

 of the dissolved gases 25.90 cubic centimetres. It will be seen 

 at once that the change ot such a dark yelloiu fluid to a colorless 

 sweet water was essential to rendering it potable. 



MECHANICAL FILTRATION. 



Uji to the present time no material has been found which is 

 practically available for filtration on a large scale except fine 

 sand. Sponge, coke, animal and wood charcoal, porous brick, 

 carbide of iron, spongy iron, and many other materials have 

 been tried, but with the result as above stated. When metallic 

 iron is used, excellent results are obtained through its chemical 

 action as a carrier of oxygen to the organic matters, which are 

 thereby oxidized and destroyed, but the water even then must 

 be subsequently filtered through sand. 



Until quite recently it has been supposed that the main bene- 

 fit of sand filtration is in the removal of suspended mud and 

 dirt, the amount of organic impurities thereby removed being 

 small. But since Pasteur discovered that the micro-organisms, 

 which are now held to be the specific germs of disease, may 

 be completely arrested by filtration through a thin porous 

 plate, a great revolution of opinion has been effected. In his 

 report for the month of May last, Dr. Frankland states that 

 the iinfiltered Thames water yielded by the method of gelatine- 

 peptone culture, 4,800 colonies of microbes per cubic centimetre 

 of water. After 2)assage throiigh sand filters at Chelsea, it 

 yielded only 59 colonies, and through those of West Middlesex 

 only 19 colonies. This is indeed astonishing, and the more so 

 when the remarkably pure water in the deep chalk^wells of Kent 

 yielded 8 colonies, and the same water by the time it reached its 

 point of supply had increased in its number of micro-organisms, 

 until 101 colonies were obtained in the culture liquid. 



At the present time, American engineers regard it as impracti- 

 calle to introduce the English system of sand-filters, on account 

 of the great expense of operating them. This has been variously 

 estimated at from $2.50 to $5.00 per day for each million gallons 



