The description is found in "The Last Night" (Ch 8). Poole has successfully swung his axe and broken into the lab. The sight is hardly one of horror, save the body lying in the middle of the floor "sorely contorted and still twitching." Otherwise, it is nothing out of the ordinary:
"The besiegers, appalled by their own riot and the stillness that had succeeded, stood back a little and peered in. There lay the cabinet before their eyes in the quiet lamplight, a good fire glowing and chattering on the hearth, the kettle singing its thin strain, a drawer or two open, papers neatly set forth on the business table, and nearer the fire, the things laid out for tea; the quietest room, you would have said, and for the glazed presses full of chemicals, the most commonplace that night in London."