Just found this:
WHAT IS THE BEST THING TO PUT IN A RADIATOR?
Special coolants are better than water (although water can be used in a pinch). Most coolants use ethylene glycol as the main ingredient. This alcohol-based fluid can raise the 212 degree boiling point of plain water by as much as 60-plus degrees. A coolant is also called an antifreeze, because it drops the freezing point down to the -80s. Inhibitors are added to coolants to control corrosion. Silicates aid in the lubrication of water pump seals. De-foamers maintain the consistency of the coolant as it's being whirred around by the impeller.
WHAT ABOUT DISTILLED WATER?
If you want to put water in your radiator, it is best to use distilled water. Distillation is a process that takes water and boils it. While the impure sediments stay down at the bottom, the steam vapor is collected. The liquid that is collected as vapor is the purest form of water you can buy. By using distilled water, you eliminate the impurities and hardness that scrapes, scars and corrodes the cooling system.