Sentence:  Then of their session ended they bid cry / With trumpet's regal sound the great result: / Toward the four winds four speedy Cherubim / Put to their mouths the sounding <WSD>alchemy</WSD>, / By herald's voice explained; the hollow Abyss / Heard far and wide, and all the host of Hell / With deafening shout returned them loud acclaim. 
Meaning of the word marked with <WSD> tags:  The causing of any sort of mysterious sudden transmutation.