MICHIGAN ACADEMY OF SCIENCE. 
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distillation. It necessarily kills all living organisms which, may be 
present, as does also boiling. We have the possibility of purifying by 
certain chemicals, and Ave have the domestic filtering apparatus, which 
is nothing more than a fine strainer. We have nature’s method, dilu¬ 
tion, which is the reduction of the percentage of obnoxious matter 
that may be present in the stream, by reason of the inflow through its 
bed, and by its tributaries, of water which is less polluted. The pol¬ 
lution under these circumstances will be further decreased by the fact 
that a large percentage of the foreign matter which comes into the water 
is heavy and will settle to the bottom. Then Ave have purification as the 
result of aeration. This is an important factor under certain condi¬ 
tions, though not of such direct importance as was once supposed. 
Vegetable growths which occur in streams also play an important part 
in the decomposition and reduction of impurities, but all of these 
processes are more or less impracticable or imperfect on the large scale, 
so finally Ave come to the practical methods of purification Avhich it is 
possible for municipalities to adopt. In.these Ave have tAvo very dis¬ 
tinct processes, which unfortunately, are designated by the same name, 
‘‘filtration,” though they have scarcely any fundamental resemblance. 
These tAvo processes comprise, first, the so-called natural or slow-sand 
method, in Avhich the agents are wholly natural, in which we make 
use of bacteria themselves to get rid of other bacteria; and, second, the 
mechanical or chemical method in which we make use of chemicals 
to precipitate the impurities, or to put them in a condition where they 
may be strained out by passing the Avater through a strainer. The 
former method is the older and was introduced in London in 1839. The 
purpose of introducing it at that time was to remove turbidity. It 
was not to aid in the bacterial purification of water, for bacteria Avere 
then unknown, but rather to make the Avater more agreeable to the 
consumer. The effect of this method Avas so satisfactory that in 1856 
its use Avas made compulsory upon the companies supplying London, 
except those avIio took Avater from subterannean sources. These filters 
were first designed by James Simpson for the Chelsea Water Company 
and Avere of practically the same type as those that are used today in 
the same process. They consisted of impervious basins, enclosing a sys¬ 
tem of under drains either of tile or brick, covered with a layer of 
gravel, which permitted the Avater to Aoav to the drains, and the gravel 
in turn was covered with a considerable depth of sand, the water being 
brought upon the top and allowed to seep through to the underdrains. 
In designing these filters, Mr. Simpson established the rate of filtra¬ 
tion to be that which Avas about the same as the ordinary Aoav of water 
through the surface layers of the ground or about six feet per twenty- 
four hours, and when so designed the filters yielded about tAvo mil¬ 
lion gallons per acre per day. After such a filter has been put in 
service there accumulates upon the surface a collection of impurities 
Avhich is known as the mat. This covers the sand grains Avith slime 
and the bacteria are entangled and held there, to such an extent that 
after a while no water Avill go through the filter, and it is found neces¬ 
sary to remove a portion of this accumulation. The filter is then 
scraped, a thin layer of sand mixed Avith bacteria is removed from 
the surface and washed, and stored until such time as the sand bed 
