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[On the black slaves at the first station]: "[…] but these men could by no stretch of imagination be called enemies. They were called criminals and the outraged law, like the bursting shells, had come to them, an insoluble mystery from over the sea. All their meager breasts panted together, the violently dilated nostrils quivered, the eyes stared stonily up-hill. They passed me within six inches, without a glance, with that complete, deathlike indifference of unhappy savages." (1.36)
Ooooh, how scary: a bunch of beaten, abused slaves are such a threat that the overseers still somehow think it necessary to chain them up. But how could these men be considered dangerous enemies.
"[…] with a large, white, rascally grin, and a glance at his charge, seemed to take me into partnership in his exalted trust. After all, I also was a part of the great cause of these high and just proceedings." (1.36)
Conrad mocks the idea of imperialism as humane by contrasting adjectives like "exalted," "high," and "just" with the brutal reality of the chained slaves. Those are some big words to describe the exploitation of cheap black labor.
"Near the same tree two more bundles of acute angles sat with their legs drawn up. One, with his chin propped on his knees, stared at nothing, in an intolerable and appalling manner, his brother phantom rested its forehead, as if overcome with a great weariness; and all about others were scattered in every pose of contorted collapse, as in some picture of a massacre of a pestilence. While I stood horror-struck, one of these creatures rose to his hands and knees, and went off on all-fours towards the river to drink. He lapped out of his hand, then sat up in the sunlight, crossing his shins in front of him, and after a time let his woolly head fall on his breastbone." (1.39-41)
Here we see the true consequences of imperialism—mistreated and overworked slaves who are left to die on their own. They're given no food, care, or medicine, and are left to die outdoors. But notice how Marlow calls them "bundles," "creatures," and phantoms"? They're treated so inhumanely that Marlow can't even see them as fully human.