92 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
that at the period of the year when the temperature tends to help the 
development of these bacteria the light is very much stronger, and 
therefore able to keep in check the stronger foe with which it has to 
contend. 
2. Another most important purifying agent is the action of the 
oxygen from the air dissolved in the water. The water is onl)^ able to 
dissolve a small quantity of air, so that we get its oxygen soon used 
up. There is little doubt, we think, that the oxidation of organic 
matter in water is partly, at least, a process of fermentation or nitri¬ 
fication, whereby the nitrogen of the ammonia is first converted into 
nitrous acid and then into nitric acid, and the presence of nitrites 
and nitrates in water is a proof of ammonia having been previously 
present. In fact, the presence of nitrites and nitrates generally in¬ 
dicates previous sewage contamination, but at such a period before 
the sample is drawn as to give time for the nitrifying organism to 
convert the nitrogen of the ammonia of the sewage into nitrous and 
nitric acids. Below a certain temperature this nitrifying organism 
ceases to perform its function and this is undoubtedly one of the 
reasons why the amount of ammonia is greater in the winter-time, 
and also for the absence of nitric acid which was not found in the 
December analyses. 
3. Possibly the most important agent of purification we have to dis¬ 
cuss, however, is filtration. The scheme in use was devised in 1834 by 
Dr. Adam Anderson, whose report to the Water Commissioners Ye 
have carefully perused, and as a natural means of filtration, y e think, it 
Y'as ingeniously planned and carefully and successfully carried out. 
There is, hoY^ever, so far as Y^e have been able to ascertain, no means 
of regulating the rate of filtration which has been recently proved to 
be a most important factor in efficient filtration; also during Y’inter, 
YTen there is severe frost for a time, and the river I0Y3 a large pait 
of the filter-beds is certain to freeze. Then when the thaY" comes, 
and a large volume of Y’ater descends, the extra piessure on the 
limited filtering area forces the water through the filter-beds at a rate 
which prohibits efficient filtration. Those of you who cross the 
bridge regularly, and watch the upper filter-bed when there is high 
tide, and see the immense rate—as shoYui by the large volume of air 
bursting up through the water right above the top of the tunnel at 
which the water enters the tunnel and forces out the air when the 
extra pressure is on, Y’ill readily understand Yhat Ye mean. 
The effective action, where due precautions are used, of a sand 
and gravel filter for purifying highly contaminated water was very well 
demonstrated in the case of Hamburg and Altona during the recent 
cholera epidemic, and is fully and carefully set forth in papers by 
Koch, of which a translation was published in the Scotsman of August 
