286 Scientific Proceedings, Royal Dublin Society. 
This would amount to nearly half the total volume of oxygen in 
the air-space, which should certainly be taken as the maximum 
allowable reduction of the oxygen in the air-supply in B. Hence 
if the polluted water to be examined be an unpurified sewage, 
it would be necessary either to dilute it to five or ten times its 
volume with tap-water, or to replenish the oxygen in the 
atmosphere of B by the addition of fresh oxygen, which can very 
easily be done in the following manner:—A store of oxygen is 
kept in an ordinary Lunge’s nitrometer over a saturated solution 
of common salt. After a reading of the volume of water which 
has risen in the graduated connecting tube of the apparatus has 
been taken, the nitrometer is connected by a short piece of 
capillary rubber tubing to 0,, and its pressure tube is raised so as 
to cause a pressure upon the oxygen a little greater than that of 
the atmosphere. ‘he stopcock 6, and that of the nitrometer are 
then opened ; and the stopcock 0, is cautiously opened, and oxygen 
is allowed to bubble up through the water in B into its air-space 
until the level of the water in the graduated tube sinks to that of 
the zero-mark on the scale. The stopcock 0, is then closed; and 
if the operation has been correctly carried out, the atmosphere in 
B will have been restored to its original composition ; and the 
volume of oxygen which the nitrometer has lost will also be a 
measure of the oxygen which had been observed by the polluted 
water in B. In this simple way the oxygen in the atmosphere of 
B may be replenished from time to time; and so the rate of 
absorption of the most highly polluted sewage may be observed 
until the sewage becomes entirely seli-purified under aerobic 
conditions. 
‘he above dimensions have all been adopted with the view of 
allowing the use of volumes of polluted water sufficiently large 
for subsequent chemical analysis, if such be necessary. I, 
however, this be not necessary, as in many cases it will not 
be, the apparatus may be reduced to one-tenth the sizes above 
given. A and B would then have a capacity of 120 to 150 cc. 
each; and the graduated tube would read to 1/100th e.c., and 
have a total capacity of 0°3 cc. Such a tube would be about 
50 mm. long, and 2 mm. bore. ‘The bulb a should have a 
capacity of 1 to 2 ec., according to whether it be required for 
the first or second of the above purposes. 
