CHEMICAL EXAMIXATIOX OF THE WATEK SUPPLY. 287 
* 
the case is also evident from the fact that while the ox}^gen required 
to oxidize the organic matter present in the water of the Washington 
Cit}^ reservoir amounts to three parts per million; in the filtered 
water onh^ two parts are required, indicating a removal tlmough fil- 
tration of oxidizable material amounting to 33.3 per cent of that 
present in the applied water. When it is borne in mind that an}^ 
considerable quantity of nitrites in water mdicates that changes due 
to livmg organisms are taking place in the water -and that, as has 
been observed, the amount of nitrites in the waters of the reservoirs 
rapidly increases in amount on standing, whereas it mcreases only 
with extreme slowness in the effluent, the decrease m the amount of 
nitrites brought about by filtration is, in this particular instance at 
least, a reasonabh^ good index of the bacterial purification of the 
water. As compared with the water of the Washington City reser- 
voir, the filtered water shows a dimmution m the amount of nitrites 
present in the water amounting to 96.4 per cent. It will be seen 
from the curves m Fig. II that a close parallelism exists between the 
several amounts of nitrites in the several reservoirs and the percent- 
age of specimens showing the colon bacillus. In this connection it 
has been pointed out by Mills ® that the vigorous nitrification taking 
place in the sand filters results not only in the burning up of nearly 
all of the Qrganic matter, but also in the destruction of pathogenic 
organisms. 
It is evident, therefore, that as nearly as can be determined from 
the results of the chemical analvses, slow sand filtration has effected 
a considerable improvement in the quality of the Potomac water. 
Unfortunatel}^, however, chemical methods are not sufficiently deli- 
cate to throw much light on the actual efficiency of sand filtration so 
far as the removal of dangerous pollution is concerned. 
THE WATER OF TAPS AXD HYDRANTS. 
As may be seen from an examination of Tables 2, 7, and 24, the 
differences in composition between the filtered water of the storage 
basin and the water delivered at various taps and hydrants are so 
slight as to be practically meaningless, in the light of our present 
chemical standards for determining the potability or pollution of a 
water. If they amount to anything at all, the slight differences 
found to exist in the composition of the filtered water of the storage 
basin and that flowing from taps and hydrants in the city are in 
favor of tap and hydrant water. The total solids and mineral mat- 
ter are slightly less in tap water than in the filtered water of the 
storage basin, and the same is true of free and albuminoid ammonia, 
nitrites, and oxygen consumed. The amount of dissolved oxygen 
a See Mason, Water Supply, New York, 1898, p. 127. 
