Alan- There are indeed many such machines. I'll just describe a few.
1. Your freezer. If you freeze salt water, the salt is excluded
from the ice. If you pour of the salt water around the ice, you'll be
left with almost salt-free ice. The trick here is to set the freezer
not too cold, so the ice doesn't feeze too rapidly. If it freezes too
rapidly, the salt can get trapped in little pockets, hard to separate
from the nearly pure ice crystals themselves. It may be best to grab
the ice out before most of the water has frozen.
2. A still. This is the traditional way to purify water. When water
is boiled, the vapor contains almost no salt. Many other impurities are
also left behind in the liquid. If the vapor then travels to a cooled
coil, it will recondense into purified water.
3. Reverse osmosis. The water is forced under pressure through tiny
pores. Electrostatic forces prevent most of the (charged) salt ions
from enetering the pores, so the water that comes through has greatly
reduced salt conentrations.
You could build your own still, or scrounge around in science
supply sources to buy one. Reverse osmosis systems should be available
from many commercial water purification supply companies.
(published on 10/22/2007)