First, let me explain the part about water. The trick here is to know what is in the water. When salt is dissolved in water, it breaks up into ions- electrically charged particles. Because they have electrical charge, they conduct electricity. For example, one type of salt is sodium chloride, with 1 atom of sodium per 1 atom of chlorine. When you dissolve this salt in water, the sodium forms positively charged ions and the chlorine forms negatively charged ions. These are pulled opposite ways by an electric field, giving a current. In very pure water the only ions are from a few water molecules that fall apart into positive hydrogen ions (H+) and negative hydroxyls (OH-). There aren't enough of these to conduct very well.
Many of the substances that dissolve best in pure water form ions, and thus conduct. Substances which don't form ions, like many cooking oils, often don't dissolve much in water anyway, and are fairly good insulators.
Now about dirty liquid air, we have to make a guess. The reason that ions dissolve well in water is that water molecules are polar- they have a negative part (the oxygen) and positive parts (the hydrogen). The ion can stick to the oppositely charged part of the water. The oxygen and nitrogen molecules which make up liquid air are not polar molecules. So we'd guess that ions don't dissolve well in liquid air, so that typical impurities in it would not conduct.
Adam (w. Mike)
(published on 08/02/06)