Aprnry— The Reservoir at Roundwood. 213 
figures being due to the experimental errors. We may draw from 
these results the conclusions that neither the unfiltered nor filtered 
water contained an appreciable quantity of fermentable organic 
matter. We may also draw the interesting conclusion that no 
nitrification took place to any appreciable extent during the space 
of seven days in the unfiltered water. I have no doubt that 
nitrifying germs did exist in the sample of unfiltered water 
experimented with. The explanation of the negative results 
of the experiment rather lies in the probable inactivity of the germs 
than in their total absence. These germs must have opportunity 
of developing and multiplying very rapidly before they will set up 
nitrification to any appreciable extent. This statement is borne 
out by one which the Royal Commission upon the Metropolitan 
Water Supply make in their Report recently published in reference 
to the action of a filter-bed. The Commissioners state :—““ The 
action of a filter-bed appears to be partly mechanical, partly vital ; 
but the mechanical action which is confined, or almost confined, to 
the holding back of the grosser substances suspended in the water, 
which was supposed until recently to be the only operation in a 
filter, is now held to be of far less importance than the vital action 
which depends on the activities of the gelatinous layer of living 
matter gradually deposited on itssurface. A new filter, composed 
of perfectly purified sand, has little or no effect in producing either 
chemical or bacteriological purification; but, in course of use, a 
layer charged with living microbes is deposited upon the surface, 
and it is by these organisms which constantly increase in number, 
and also penetrate the sand to a slight distance, that both the nitri- 
fication of organic matter and the arrest of other microbes is 
effected.” 
Referring again to the experiment of keeping the samples of 
unfiltered water out of contact with air, I should like to add that I 
believe, from other experiments I have made, that a negative result 
in the search for nitrification would not have been obtained in the 
case had I been able to keep the sample for a longer period. In 
time I have no doubt that the ammonia would have been completely 
nitrified, the organism deriving the carbon for their metabolism 
from the colouring matters in the water ; but this is a question re- 
quiring further experiment, and which I must leave for a further 
communication. 
