Specification :

Bath salts provide a variety of benefits to a bather. Salts change the osmotic balance of the water so that less water is absorbed by the skin via osmosis. This reduces the " pruning" or " wrinkling" effect of prolonged exposure of skin to fresh water.
Some bath salts such as phosphates have a detergent action which softens calloused skin and aids in exfoliation.
Some bath salts act as water softeners and change the way soap rinses. Some confusion may arise after a first experience with soft water. Hard water does not lather well with soap and can leave a sticky feeling. Soft water lathers better than hard water but feels slippery for a longer time during rinsing of soap, even though the soap is coming off faster, because the soap remains soluble.
High concentrations of salts increase the density of the water and increase buoyancy which makes the body feel lighter in the bath. Very high concentrations of salts in water are used in flotation therapy.

( from wikipedia)