He put the glass to his lips, and drank at one gulp. A cry followed; he reeled, staggered, clutched at the table and held on, staring with injected eyes, gasping with open mouth; and as I looked there came, I thought, a change—he seemed to swell—his face became suddenly black and the features seemed to melt and alter—and at the next moment, I had sprung to my feet and leaped back against the wall, my arm raised to shield me from that prodigy, my mind submerged in terror.

“O God!” I screamed, and “O God!” again and again; for there before my eyes—pale and shaken, and half fainting, and groping before him with his hands, like a man restored from death—there stood Henry Jekyll! 

This quotation appears in Chapter 9, “Dr. Lanyon’s Narrative,” as Lanyon describes the moment when Hyde, drinking the potion whose ingredients Lanyon procured from Jekyll’s laboratory, transforms himself back into Jekyll. Lanyon, who earlier ridicules Jekyll’s experiments as “unscientific balderdash," now sees the proof of Jekyll’s success. The sight so horrifies him that he dies shortly after this scene. The transformation constitutes the climactic moment in the story, when all the questions about Jekyll’s relationship to Hyde suddenly come to a resolution.

Stevenson heightens the effect of his climax by describing the scene in intensely vivid language. When he depicts Hyde as “staring with injected eyes” and suggests the dreadful contortions of his features as they “melt and alter," he superbly evokes the ghastliness of the moment of transformation. As this passage emphasizes, the true horror of Jekyll and Hyde’s secret is not that they are two sides of the same person, each persona able to assert itself at will, but that each is actually trapped within the grip of the other, fighting for dominance. The transformation process appears fittingly violent and ravaging, causing the metamorphosing body to “reel," “stagger," and “gasp.” Indeed, by this point in the novel, Jekyll is losing ground to Hyde, and, correspondingly, emerges “half fainting," as if “restored from death."