"The Ring Of Wax"
Riddler uses a universal wax solvent he smuggled over from Europe in order to obtain a book entitled "Lost Treasure of the Incas". Inside, the secret location of the treasure is revealed.
When Riddler has Batman and Robin in his clutches he could easily bump them off but he explains why he prefers to kill them slowly. "Why kill them fast when watching them die slowly is more entertaining? I want to savor every delicious moment of this."
Guess which substance the Dynamic Duo were almost immersed in this time!