|MadSci Network: Chemistry|
Condensation occurs when humid atmosphere finds itself chilled to a temperature where it cannot hold all of its water vapour, and gets rid of the excess when liquid droplets form on a local surface. Salt -- pure sodium chloride -- has some affinity for water vapour, and will remove it from humid air. That is why salt left in the open air usually becomes damp and lumpy. But the affinity of pure sodium chloride for air is not so very great. Table salt, the sort you get in your salt shaker, is not pure sodium chloride. If it were, it would not be a very satisfactory product, because it would gradually get damp and lumpy and not pour freely. To help it to work properly as a dry fine crystalline product that pours freely, other materials are added. The formulation of table salt varies from country to country, according to the climate and local regulations. In large countries like the US or Australia, it probably even varies from region to region. But one of the additives is something to remove water vapour from air a bit more efficiently than salt itself, and thus stop it from getting damp and caking. I believe cornflour (corn starch?) is commonly used for this purpose. You might be able to tell by looking at the ingredient list on a pack of table salt. In Tennessee I believe you have a very humid climate in the warmer months, and so an additive of this sort with a large capacity for water vapour would be needed. [Editors note: My container of Morton's Iodized Salt contains calcium silicate and dextrose, both of which would have the effect Dr Christie describes...] So if there is a layer of (table) salt under your glass, the salt itself, or more likely the additives, will remove enough of the water vapour that liquid water will not get onto the table.
Try the links in the MadSci Library for more information on Chemistry.