The attack did not begin with battering rams; it began with the insidious hiss of chemical dissolving.
Through the narrow drainage trench that carried water drops from the outer gatehouse toward the Chasm Flues, a dense, purple liquid began to creep along the dark rock floor. The test chemical rolled forward like heavy syrup, giving off clouds of pale green vapor that curled around the base of the Great Iron Arch, filling the air with the stinging tang of boiled copper and strong etching acid.
Qiao Ming knelt on the inner threshold, his gauntlets of greased shark-skin leather drawn up to his elbows. He brought his horn check lamp within three inches of the drainage weir. The purple syrup had reached the main mortar joint between the western pilaster and the living bedrock.
Where the test chemical touched the stone, the dense black dark rock began to boil.
Tiny, violent bubbles of carbon dioxide and sulfur dioxide erupted from the stone matrix. That turned the hard crystal-clear stone into a soft, greenish-grey paste that crumbled like wet sand under the acid-burning attack. The void-liquid silver aqua fortis was eating into the foundation at a rate of nearly an inch an hour.
