Very interesting. Did you find any noticeable difference between the longevity of its heating as compared with water? The reason I ask is that the specific heat of water is higher than that of wheat so water should retain the heat longer.

BTW, I'm familiar with hot water bottles, they were all the rage when I was a kid before electric blankets became commonplace.

P.S.: I can't say I ever had one leak but I reckon I'd have come close. After some use, they'd start to perish around the filling point/screw stopper. The idea was to always keep an eye on it and once one noticed the first signs of perishing then not to gamble too long before replacing it.