Brogue Wiki
a glass dewar of caustic gas
the dewar shatters and pressurized caustic gas explodes outward!

Dewars are glass containers of extremely pressurized gas, denoted by the "&" symbol. There are four types of dewars in the game, each containing a harmful gas (caustic gas, paralytic gas, confusion gas, or methane gas), and can be identified by either their color or by examining them. Dewars pose no immediate threat to the player, unless they are directly or indirectly affected by an attack.

Once a dewar is struck by an attack, it shatters (leaving shattered glass tiles) and releases 20,000 units of its gas type. Because of this extreme volume of gas contained in the dewar (20x more than an equivalent potion), the resulting cloud expands very rapidly, covers a very large area if unobstructed, and can linger for quite a while. In enclosed rooms, they can be even more dangerous, taking hundreds of turns to disperse, depending on the size of the room.

Because of the large amount of gas, dewars can be quite effective at affecting large numbers of mobs. A paralysis dewar in a small room can immobilize any monsters in that room for an extremely long time, leaving the player free to explore without interruption. A confusion dewar near lava can send hordes of mobs plummeting into the molten lake. Due to the threat they pose, though, it's recommended that any player who wants to utilize a dewar and the gas it contains should do so from a safe distance away and shatter the dewar via ranged means.