32 Mr. H. M. Witt on the Power possessed by Porous Media 



power of charcoal to remove organic principles from solution in 

 water, its superiority over sand is more especially observable in 

 this respect; no weighable quantity of organic matter being 

 separated by the sand, whilst 15 per cent, of the quantity present 

 was removed by the charcoal. 



Nevertheless the case is quite altered when more impure water 

 is filtered through the same medium ; for instance, it will be 

 remembered that sand does remove organic matter from water 

 when the quantity present is somewhat greater. Thus, in 

 Tables I., II. and III. it is shown that at Chelsea, where the 

 quantity of organic matter present in the water rose to 2*375 

 grs. in the gallon, then the sand removed 20*46 per cent ; when 

 the quantity present amounted to 405 grs., 66*66 per cent. ; 

 and lastly, when to 9*909 grs., no less than 95*23 per cent, was 

 removed ! 



Now this is a most striking and important result in a hygienic 

 point of view. The evil consequences said to result from the 

 water supply of London at present in use are mainly attributed 

 to the organic matter present in the water ; but these analyses 

 show that the more impure the water (within certain limits), the 

 greater is the power of porous filtering media to remove these 

 impurities, probably from the more perfect contact between the 

 filtering medium and the impurities present. It is obvious that 

 this would not go on ad infinitum ; it must not be supposed that 

 I wish it to be inferred that the more organic matter there is 

 present in a water, the more would necessarily be removed by 

 sand filtration ; quite the contrary, it is highly probable that 

 if the quantity had increased much beyond what was contained 

 at the time of the performance of these experiments, a consider- 

 able amount might have passed through unrestrained ; all I wish 

 to be understood is, that within the limits of the quantities present 

 when these experiments were made, sand was found incapable of 

 removing any appreciable quantity of organic matter when the 

 amount originally present in the water was exceedingly small; 

 but that in the same ratio as the quantity present increased, the 

 proportion separated rose. 



Now the power of charcoal, I believe, increases in a similar 

 manner ; but no experiments could conveniently be made on the 

 large scale at Chelsea to enable me to draw a strict comparison 

 between the effects of sand and charcoal upon the more impure 

 water there, similar to what had been made at Kingston with the 

 comparatively pure water ; therefore to enable me to adduce ex- 

 perimental evidence on this point, a comparative experiment was 

 made on the small scale with an artificially prepared impure 

 water. 



This water was prepared as follows : — To the sample of Thames 



