MICHIGAN ACADEMY OP SCIENCE. 
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removed. In Europe, where this process is used, and where they often 
deal with very impure water, typhoid fever is of such rare occurrence 
that a case of it is lmiled with delight by the students of medicine. 
The death rates in American cities where filters have been introduced 
have shown a most remarkable drop. In Albany the death rate from 
typhoid fever dropped 75 per cent upon the introduction of filtered 
water. This water was taken from the Hudson polluted with the 
sewage of Schenectady only a few miles above. 
It was my privilege to visit the Lawrence Experiment station in 
September, 1895, and I saw there sewage from the main sewer of the 
city of Lawrence, a city of over 40,000 inhabitants, being passed through 
about live feet of pea gravel at a rate of a quarter of a million gallons 
per acre per day, an effluent being obtained of sufficient purity to offer 
no offense if turned into a stream whose waters Avere used for any but 
drinking and culinary purposes, saw this effluent then passed through 
about five feet of sand very similar to our lake sand, at a rate of two 
and one-half million gallons per acre per day and saw the final effluent 
of such purity that the assistants in the laboratory had no hesitation 
in drinking it, its composition being about that of the best spring 
water. 
So we water supply engineers have a good deal of confidence in our 
ability to purify moderately bad water and render it safe. There are, 
however, some limitations to the slow sand process. In the case of 
wafers whose turbidity contains no clay, if is found that the slow sand 
filter works well, but if clay is present it soon fills the pores of the filter 
and cuts off the flow, and it is very difficult to remove a clay turbidity 
by simple sedimentation. At times in the spring when the Detroit 
water is turbid the suspended matter is made up of clay and sand, 
the sand turbidity will settle out in a few hours, but thirty days will 
not remove the clay. So where you have a water bearing clay it becomes 
almost impossible to deal with it by the slow sand process. A feature 
of the slow sand filter is that while turbidity may go through, it will 
still do its work in removing bacteria, so a turbid effluent is not an 
indication of defective purification with this process. 
We now come to the other process, that known as the mechanical 
filter, sometimes spoken of as the mechanical process, sometimes known 
as the American method, and perhaps more properly called the rapid 
method. These filters appeared about 1887, and were originally designed 
for use in paper mills in Maine to take out of water those substances 
which were detrimental to the manufacture of paper. They consisted of 
tubs containing coarse sand through which the water was passed. The 
water coming out was clear and very attractive, and it was thought these 
filters would be a desirable adjunct to water supply systems. A great 
many of them were put into hotels and installed in small villages 
throughout the country. They removed turbidity and a good many 
supposed they would remove disease infection. The water was passed 
through this sand at very high rates of speed, 100 times as rapidly as it 
passed through in the slow sand process, and it was soon demonstrated 
that little bacterial purification resulted. The next thing that Avas sug¬ 
gested was the use of chemicals in connection with this rapid filter. 
Chemicals had been used in Egypt many years for the purification of 
the water of the river Nile, and the suggestion Avas taken up by some 
