Why Cowan didn’t identify the black powder
James Cowan was given the particle brush found in Keane’s suitcase and asked to identify the black powder that could be shaken from its bristles.
Cowan only tested for common poisons.
“At his (PCC Sutherland’s) request I carried out analyses on those specimens, but was unable to find any signs of any common poison in any of them. I tested for common poisons. Cyanides, alkaloids, barbiturates, carbolic acid .. “
Taken from Cowan’s deposition.
He didn’t test for graphite.