Hydrosion shells, called fizzers by Commander Root, are one of Foaly's many inventions. They were built to be miniature fire extinguishers, and could compress up to half a gallon of water into a small space. When thrown at the flames, the impact would reverse the compression, and the water would be sprayed at the fire. This method was more effective at extinguishing fires than pouring a hundred gallons of water.
As with most fairy technology, Hydrosion shells are completely recyclable; putting one onto a pile of snow (and presumably water) automatically activates the compressors refilling the device.
When Artemis rescues his father, he fills a fizzer with his own blood and instructs Butler to shoot his father with it, to make it look like he had shot his father dead.