This essay’s principal objective is to examine how, when confronted with a case of possible criminal poisoning, early nineteenth-century English toxicologists sought to generate and to represent their evidence in the courtroom. Its contention is that in both these activities toxicologists were inextricably engaged in a complex communicative exercise. On the one hand, they distanced themselves from the instabilities of language, styling themselves as testifiers to fact alone. But at the same time, they saw themselves as deeply implicated in the difficulties of forging a coherent signifying system out of a disparate collection of signs that in themselves bore no intrinsic meaning. The analysis has three main components: first, to suggest why criminal poisoning featured so prominently in the burgeoning legal literature on evidence which provided the framework for expert testimony in English courts; next, to show that the scientific evidence offered by toxicologists in poisoning cases can be usefully understood as a form of (unstable) language; and finally, to suggest that this recourse to signs informed the toxicologist’s encounter with another type of courtroom expert—the legal advocate—who was equally (though differently) interested in manipulating signs in order to construct (and deconstruct) legally sanctioned proof.