Chemistry in the Industries 277
ganese ore. The flotation of phosphates is, by far, the greatest
application of the flotation process to non-metallics. In
Florida ɪɪ,ooo tons per day are recovered from deposits at
one time considered worthless. From the standpoint of
chemical engineering a novel adaptation of flotation has
come in the separation of potassium chloride from sodium
chloride. The crystals are separated from each other in a
saturated brine made up from the crude mixed salts as they
occur in natural saline lakes.
In the field of sanitary engineering appears the problem
of supplying safe and relatively soft water which will be used
in the municipality for home consumption as well as for the
industry. Good water for drinking purposes has doubtless
been appreciated by the human race from time immemorial.
Among primitive peoples the question of water supply was
never of passing importance, except in arid regions. Recent
archeological explorations in India have shown that five
thousand years ago these people were living in well-built
cities provided with well-constructed water conduits covered
with marble slabs. The ancient water tanks in Arabia may
have been built as early as 6oo b.c. The infiltration galleries
for collecting ground water at Athens were probably con-
structed two thousand years ago. Probably no more elaborate
system of public water supply was provided for any ancient
city than that of Rome. Three groups of springs in the vol-
canic plain on the left bank of the Tiber supplied the water
to be conveyed by aqueducts to the city.
The requirements for municipal water supplies are that
there should be no disease organisms present, that the water
should be clear, colorless, and odorless; that it should be
reasonably soft and non-corrosive; that it should be free from
objectionable gases and minerals; and that it should be
plentiful and cheap.