92 ILLINOIS ACADEMY OF SCIENCE 
educative work incident to their promotional activities, and it 
is perhaps pardonable that sometimes their zealousness in mak- 
ing sales warped their better engineering judgment. 
As the basic patents on rapid sand filters expired, rapid sand 
filters were built more and more according to the designs of 
technically trained engineers, and we find the best examples of 
filter practice among filter plants installed in this way. 
As an indication of the rapid progress in the purification of 
the public water supplies in this country, it may be stated that 
in 1892 not over 500,000 people in the United States were 
supplied with filtered water and most of the filters were of 
very questionable efficiency. According to figures compiled 
by Johnson, about 2,000,000 people in the United States were 
supplied with filtered water in 1900. At the present time ap- 
proximately 20,000,000 people are supplied with filtered water, 
and these plants built since the earlier period show a marked 
improvement in both design and operation. 
Striking as the above figures are, they do not tell the whole 
story of progress towards securing pure water supplies, be- 
Cause many cities have abandoned impure supplies in favor of 
supplies naturally pure, and others have protected existing sup- 
plies by storage and watershed patrol and diversion of sewage 
in a manner to adequately protect their purity. 
In the year 1906, a hitherto unused means for protecting 
the purity of water supplies was developed, namely sterilization 
by means of hypochlorites. It was found that a very high 
bacterial reduction in a water could be obtained by the applica- 
tion of six to twelve pounds of hypochlorite of normal strength 
to a million gallons of water. The means for applying the hy- 
pochlorite are very simple and so there became available a 
method for treating any water supply so as to render it at least 
safe at a trifling expense. Soon afterward chlorine gas was 
used for sterilizing water, with effects equal to those obtained 
by means of liquid chlorine. Chlorine did not come into gen- 
eral use, or threaten to displace hypochlorite until within the 
past few years, because only recently have reliable and effective 
means been devised for applying the chlorine gas to the water. 
The use of chlorine is now generally regarded as more satis- 
