___ salt

EPSOM

This magnesium-rich compound is often dissolved in bathwater to soothe sore muscles or as a laxative in medicine.

The clue "___ salt" is looking for a type of salt often used in baths for soothing muscles. "Epsom" fits because Epsom salt is a well-known magnesium sulfate compound, named after the town of Epsom in England where it was originally discovered. The blank suggests a proper noun, and "Epsom" is the most common answer for this kind of salt.