A series of essays investigates toxic agents by combining natural history, dissection, and experiment to explain how poisons act on the body. Several pieces focus on venomous animals, presenting anatomical observations and practical tests of venom delivery and effects, while others analyze poisonous minerals and plants, the pharmacology of opium, and noxious exhalations from earth, air, and water. The author links observed symptoms to physiological mechanisms and argues for mechanical and mathematical reasoning in medical explanation. Practical implications for diagnosis and treatment are suggested through close attention to bodily structure, fluid motions, and the physical properties of toxic substances.