Don't know if anyone has used rock sulphur but after a discussion on another board thought I'd try some especially as its quite hot at the moment:
"Natural rock sulphur is known to aid a dog's health and condition. Add a small amount of rock sulphur to your dog's drinking water. Helps cool your dog in hot weather and heal skin problems. Good for travelling or at times when your dog may suffer from dehydration."
Mind you true to form Wilf has already fished it out the bowl (even though he was eating a pig's ear at the time and thought he'd save it for later on LOL)