ghost dust

"Dresden," he called. "The dust!"
I fought my way forward, through the wind, shoved my fist into Agatha's weapon-arm, and shook loose some of the ghost dust from the leather sack.
Upon contact with her immaterial flesh, the ghost dust flared into blazing motes of scarlet light. Agatha screamed and jerked back, but her arm remained in place as firmly as if it had been set in concrete.
"Benson!" Agatha shrieked. "Benson! Hush little baby!" And then she simply tore herself away from her arm at the shoulder, leaving her spirit flesh behind, and vanished. The arm and hatchet collapsed to the floor in a sudden spatter of clear, semifluid gelatin, the remnants of spirit-flesh when the spirit was gone, ectoplasm that would swiftly evaporate.


Grave Peril Chapter 2, Page 16-18