THE CONTAMINATION AND PURIFICATION OF STREAMS I4 5 
River, below Albany. It has become a matter of great importance 
to know whether freezing does purify such ice and render it safe. 
The subject has been mosf carefully investigated, with the follow- 
ing conclusion. Ice does in a measure purify itself in freezing, but 
not wholly. If typhoid bacilli are in the water, they may be found 
in the loose snow ice at the top of the frozen layer, but there are 
very few, if any, in the clear ice below. After the ice has been 
stored for a while the typhoid bacilli become less and less abund- 
ant, and after a few weeks they practically disappear. Even after 
months of freezing, however, a few may sometimes be found, so 
that no ice from contaminated water can be guaranteed as abso- 
lutely free from them even after six months’ storage. But the 
number that resists this storage is so extremely small that the ice 
is as pure as filtered water. No cases of typhoid fever have been 
definitely traced to such a source, though one or two doubtful 
cases have been so attributed. In general, then, it appears that 
stored ice is safe, provided it is free from snow ice and has been 
stored at least two or three months. 
II. THE CONTAMINATION AND PURIFICATION 
OF STREAMS 
The sewage contamination of streams has been increasing year 
by year, until many a stream that was clear and limpid thirty years 
ago is now a vile collection of filth Until some other means of 
disposing of sewage is generally adopted, this pollution must con- 
tinue to increase. There has been a widely held belief that run- 
ning water purifies itself, and that these streams rapidly become 
free from their pollution. This is partly correct and partly errone- 
ous. A stream does not purify itself by running, but there is 
always a tendency for water to become pure, and in time sewage 
contamination quite disappears from water, whether running or 
stagnant. The best studied example of this is in the Chicago 
Drainage Canal. Recently the city of Chicago converted the 
10 
