Science indeed will tell you that the very notion of any such poison is flagrantly absurd, and that such a toxic action is against all the laws of nature.
But a scientific disquisition is unnecessary.

For our present needs arguments of common sense should abundantly suffice.

This poison--this white powder--was said to be a secret of the Borgias.

If that is so, by what Borgia was the secret of its existence ever divulged?
Or, if it never was divulged, how comes it to be known that a poison so secret, and working at such distances of time, was ever wielded by them?
The very nature of its alleged action was such as utterly to conceal the hand that had administered it; yet here, on the first recorded occasion of its alleged use, it was more or less common knowledge if Giovio and Guicciardini are to be believed! Sagredo( 1) says that Djem died at Terracina three days after having been consigned to Charles VIII, of poison administered by Alexander, to whom Bajazet had promised a large sum of money for the deed.

The same is practically Giovio's statement, save that Giovio causes him to die at a later date and at Gaeta; Guicciardini and Corio tell a similar story, but inform us that he died in Naples.
1 In Mem.