From forensic toxicology to biological chemistry: Normal arsenic and the hazards of sensitivity during the nineteenth century.

This paper reviews the cultural meanings, social uses and circulations of arsenic in different legal, medical and popular settings. The focus is on nineteenth-century France. In the first section, I review the advent of the Marsh test for arsenic, which is commonly regarded as a milestone in the history of toxicology. I claim that the high sensitivity of the Marsh test introduced puzzling problems for forensic doctors, the most disturbing one being the so-called 'normal arsenic.' I reconstruct early research on normal arsenic and the ensuing controversies in courts, academies and salons. A report from the French Academy of Science converted normal arsenic from a big discovery to an experimental mistake. In the next section, I study how these disturbing conclusions were perceived by toxicologists all over Europe and how normal arsenic disappeared from view by the middle of the nineteenth century. Finally, I review the return of normal arsenic thanks to Armand Gautier and Gabriel Bertrand, who introduced an innovative research framework and so prompted the displacement of arsenic from criminal toxicology to pharmacology and nutrition science. The last section will also show that the issue of normal arsenic was recaptured in public debates concerning criminal poisoning at the beginning of the twentieth century.