ON WATER-FILTERS. 
39 
possessed by the material is exerted for this period upon the 
water, instead of during the very short time that water takes in 
passing through a filtering medium. On the other hand, this 
filter being constructed to yield a flow of from half to several 
gallons per minute, there is the disadvantage attending such 
rapid filtration that the water is subjected to the purifying 
action of the material for such an exceedingly short period. How- 
ever, admitting the correctness of these remarks, it is obvious 
that, by only drawing off small quantities at a time — say not 
more than a pint or two from the smallest size — the advantage 
of the arrangement remains in full. 
An advantage spoken of by the company, the importance 
of which will be generally undisputed, is not, however, felt by 
ourselves to be very material : for if the water is properly pu- 
rified, storage in a close vessel of earthenware or glass, as is 
practised in other filters, will not diminish its freshness or its 
wholesomeness. An objection, in our opinion, to the cistern 
arrangement, also lies in the very rapid passage through the filter 
which is required to maintain the comparatively large stream 
issuing from the tap, since, in order to allow of this, the me- 
chanical filtration is very liable to be imperfect. 
The only material employed by the London Water Purifying 
Company is, we believe, animal charcoal. The high purifying 
effects, therefore, of their filters is not to be wondered at, 
though not the less, therefore, of the most material value. 
Of the filtering capacity, in the mechanical and proper meaning 
of the term, of HanchelPs filter, we cannot speak very positively ; 
and we do not, therefore, challenge any statement as to its ex- 
cellence or otherwise. Its purifying effects are exceedingly 
striking. All the objectionable sensible qualities of water due 
to organic matter are removed from the water, and chemical 
examination shows that nearly the whole of the organic matter 
is removed ; together with this nearly all the dissolved gases 
and the dissolved chalk. The latter effects show the powers of 
the filter, and render the w r ater better fitted for culinary and 
domestic properties ; but they are hardly desirable when the 
water is for drinking, as it becomes somewhat insipid, through 
the loss of its chalk and dissolved gases. 
Of the two filters we have yet to notice, we have found the 
one we are going next to describe to lie intermediate in power 
of purifying water from dissolved organic matters. Its filtering 
powers seem to be excellent, and we are inclined to think that 
it admits of being more surely constructed, so as to filter effec- 
tually in every case, than other filters. Our reason for so 
thinking is, that being formed of a slab of the filtering material 
cemented into a vessel, so as to divide it into two chambers, it 
seems only necessary to select slabs free from flaws to make it 
