sluggish breeze was blowing out of the swamp, drawn by the cooler uplands beyond the fort; it took the smoke and sent it rolling toward the packed and struggling mass on the earthworks.

Out on the battlefield, Nahali stiffened suddenly, fell tearing convulsively at their bodies. The beating rain washed the soot down onto them harder and harder, streaked it away, left a dull film over the reptilian skins, the scaly breast-plates. More and more of them fell as the smoke rolled thicker, fed by the blackened madman under the liha-tree, until only Legionnaires were left standing in its path, staring dumbly at the stricken swamp-rats.

The squirming bodies stilled in death. Hundreds more, out on the edges of the smoke, seeing their comrades die, fled back into the swamp. The earthworks were cleared. Ian MacIan gave one wild shout that carried clear to the fort. Then he collapsed, crouched shivering beside the unconscious Lehn, babbling incoherently.

Thekla, strained on tiptoe under the tree-branch, had stopped smiling.

The fever-mists rolled away at last. MacIan woke to see Lehn's pink young face, rather less pink than usual, bending over him.

Lehn's hand came out awkwardly. "I'm sorry, MacIan. Thekla told me; I made him. I should have known." His grey eyes were ashamed. MacIan smiled and gripped his hand with what strength the fever had left him.

"My own fault, boy. Forget it."

Lehn sat down on the bed. "What did you do to the swamp-rats?" he demanded eagerly. "They all have a coating as though they'd been dipped in paraffin!"

MacIan chuckled. "In a way, they were. You know how they breathe; each skin cell forming a miniature electrolysis plant to extract oxygen from water. Well, it extracts hydrogen too, naturally, and the hydrogen is continually being given off, just as we give off carbon dioxide.

"Black smoke means soot, soot means carbon. Carbon plus hydrogen forms various waxy hydrocarbons. Wax is impervious to both water and air. So when the oily soot from the smoke united with the hydrogen exuded from the Nahali's bodies, it sealed away the life-giving water from the skin-cells. They literally smothered to death, like an Earthly ant doused with powder."

Lehn nodded. He was quiet for a long time, his eyes on the sick-bay's well-scrubbed floor. At length, he said:

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