414 BIOLOGICAL STUDY OF TAP WATER. 
few minutes, will afford an abundant supply of sediment. The other 
plan is to open the tap to the full extent and allow it to run for a 
short time, so as to stir up whatever sediment may be in the pipe ; 
then a tall glass cylinder is filled, and a watch-glass attached to a 
piece of platinum wire, by which it can be raised, is let down to the 
bottom of the vessel. The whole lightly covered is put aside for 24 
hours to allow it to settle, and after this the water is siphoned off 
almost down to the watch-glass, which can then be raised without 
disturbing the sediment which it contains. This latter method 
possesses the advantage that the same quantity of water is always 
taken, and thus the amounts of sediment at different times can be 
compared ; while it is almost impossible to fix a tap to run continu- 
ously at a given rate, owing mainly to variation in the pressure of 
the water in the pipes. 
A little of the sediment obtained in either of these ways was 
transferred by a pipette to a slide, and examined with a Hartnack 
Objective No. 8 and No. 4 Eyepiece. This combination has a 
magnifying power quite high enough for diagnosing the most of the 
forms ; though on one or two occasions a No. 10 Immersion was 
used. 
The actual amount of suspended matter present in any definite 
quantity of the water varies very considerably, and depends upon 
several conditions, among which some of the most noticeable are the 
season of the year, the amount being greater in winter and spring 
than at any other time; the prevalence of stormy weather; the 
quarter of the city from which the water is taken; and the tap 
itself ; for, if the water be drawn from a pipe which is seldom used, 
it is sure to contain more sediment than that from one in constant 
use, as it settles when allowed to rest for some time. There is no 
doubt also that organisms are often found in the mains which are not 
found in water taken directly from the lake. This, together with the 
fact that the number of individuals of some species is greater in the 
water of the mains than in that of the lake, may be explained on 
the supposition that the former habitat affords them a better food- 
supply, consequently they multiply more rapidly. Tne exclusion of 
light also seems favorable to the development of certain forms. 
Another marked result of these investigations has been the conclu- 
sion, that many of the forms seem to have a preference for certain 
times of the year, being much more abundant then than at any other 
