This essay explores how the enhanced sensitivity of chemical tests sometimes produced unforeseen and puzzling problems in nineteenth-century toxicology. It focuses on the earliest uses of the Marsh test for arsenic and the controversy surrounding “normal arsenic”—that is, the existence of traces of arsenic in healthy human bodies. The essay follows the circulation of the Marsh test in French toxicology and its appearance in the academy, the laboratory, and the courtroom. The new chemical tests could detect very small quantities of (...) poison, but their high sensitivity also offered new opportunities for imaginative defense attorneys to undermine the credibility of expert witnesses. In this context, toxicologists had to dispel the uncertainty associated with the new method and come up with arguments to refute the many possible criticisms of their findings, among them the appeal to normal arsenic. Meanwhile, new descriptions of animal experiments, autopsies, and cases of poisoning produced a steady flow of empirical data, sometimes supporting but in many cases questioning previous conclusions about the reliability of the chemical tests. This challenging scenario provides many clues about the complex interaction between science and the law in the nineteenth century, particularly how expert authority, credibility, and trustworthiness were constructed, and frequently challenged, in the courtroom. (shrink)