Actually, that can't be proved -- nor disproved -- right now, because they're not sure if the hair samples they have come from Napoleon.
Here's the latest:
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3 - Was Napoleon poisoned or did he die of stomach cancer?
Antommarchi's autopsy report is very complete and shows Napoleon's general state of health at his death, notably a chronic stomach ulcer and
pulmonary lesions linked to tuberculosis. Cancer cannot be diagnosed because of a lack of histological evidence from the stomach lining. At any rate,
one does not die 'of cancer', one dies of the effects of the cancer on the organism.
Analysis of the emperor's hair and the discovery of high level of arsenic therein poses several questions. But it is intellectually impossible to
accept the theory of death by arsenic poisoning.
First of all, we can never be 100% certain that the hairs analysed come from Napoleon. Furthermore, the level of arsenic could be interpreted in
different ways, notably the methods of analysis and the ways of calculating the levels used by the toxicologists (numbers obtained weighed against the
number of hairs analysed: in fact, very few hairs have been analysed. Whilst presence of arsenic cannot be explained arguing from its external use (in
cosmetics, for example), we still do not know where the arsenic came from, and it could have come from many sources. The hairs on the head of the
people in Napoleon's entourage could also have a high arsenic content.
Finally, to pass from toxicological results to a poisoning theory, then to a voluntary criminal act is very difficult. Indeed, one cannot establish a
theory, accepting certain elements of the correspondence of one of the protagonists whilst eliminating other elements two paragraphs further on which
contradict this position.
The only certainties thus are, Napoleon's general state of health was very poor and no direct cause of death can be determined accurately. This is
the only satisfactory conclusion from an ontological point of view, both for the scientist and for the historian. A deeply held conviction may be the
starting point of an investigation but certainly not its conclusion.