History of the Medical Use of Silver
http://tse.colloidalsilverkillsviruses.com/pdf/history.pdf

excerpt:
During the early pioneer days on the North American
continent, when there was no refrigeration and water needed
to be transported long distances, it was common practice to
drop silver coins into the transport vessel to preserve water.
This practice also was used to preserve milk and prevent
spoilage, without knowledge that it was the prevention of
bacterial growth that caused the effect.