These salts are deliquescent, meaning that they attract water. They attract so much water that, if they are exposed to even the slightest bit of water vapor in the atmosphere, they will absorb it into their crystal structure and melt themselves into a brine.
This specimen was stored in a vial that was *almost* airtight — but not quite. The water you see in the vial is the result of 100 years of tiny amounts of water vapor seeping into the vial and being absorbed by the salt crystals, which have begun to melt.
This specimen is from Strassfurt, Germany.
Bryn Mawr Vaux Collection, Sample #1873