I have seen this on a BBC programme called "Tomorrow's World" back in the late 70's or early 80's.
I saw it first used as the contents of a storage heater to be used in a Truck (for when the driver sleeps in the cabin).
I saw it later as a row of gallon containers in a greenhouse to help keep the internal temperature constant.
I believe that the melting point was raised to over 20 degrees celcius (70 F) and maybe quite a lot higher.
If you have any idea what this stuff is, I would like to know!