ON WATER-FILTERS. 
41 
would be found too friable, not to mention that after a short 
time it would have to be renewed. One slight objection to the 
action of the magnetic carbide upon water is that it is apt to 
render it very slightly contaminated with iron if the water con- 
tains much carbonic acid. 
It will thus be seen that, in the magnetic carbide filter of 
Spencer, the cistern-filter of Danchell, and the silicated carbon 
filter, we may possess with tolerable certainty the means of 
freeing water from matters injurious to health. The uncertainty 
lies in the fact that the particular filter used by a person may 
be imperfect as a mechanical filterer, and may have become in- 
efficient as a purifier from dissolved organic matters, unless, as 
regards the latter point, it be a Spencer’s filter, and have only 
been used with water of tolerable clearness (such as that sup- 
plied to London), and not largely charged with carbonate of 
lime in solution. Hence arises the propriety of having a 
filter examined after being placed in a house, in order to test its 
efficiency. This ought to be done by the filter-sellers. 
