The writer uses imagery in the given sentence to paint an image of the current scenario in the reader's head.
Imagery by definition is a literary device used in poetry, novels, proses and other writing which uses vivid textual description that appeals to a readers' senses in a bid to create an image or idea in their head.
On this note, the writer uses imagery in the given sentence to; create the series of events which lead to the acid eating their fingers off in the reader's head.
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