Henry sweats out of pure nervousness. The fight is about to begin, and before he is ready or can consciously decide, he lowers his rifle and fires the first shot of the battle. Henry then loses concern for himself, and becomes "not a man but a member." Whatever he was part of, it was in a critical state, and he was part of its desire. The noise of the firing reassures Henry in his actions.
The furious haste and noise make the atmosphere even more confusing: sweat blisters, his eyes are hot, and the blasts burn in his ears. He is not fighting the enemy of men so much as the swirling battle phantoms that surround him.