Page "The Adventure of the Devil's Foot" Paragraph 14

It also seems clear to Holmes that Mortimer Tregennis was guilty of using the poison on his siblings, but who killed him?

Holmes ’ s investigation has made that quite clear.

It is Dr. Sterndale.

He left physical evidence at the vicarage clearly implicating him.

All that Holmes does not know is why Sterndale did it.

Sterndale explains that he loved Brenda for years ( but had been unable to marry her because of the current marriage laws which prevented him from divorcing his wife even though she abandoned him years ago ) and killed Mortimer for what he had done.

It also turns out that he knew about the poison long before Holmes.

It is called Radix pedis diaboli (“ Devil ’ s-foot root ” in Latin ), and he brought it from Africa as a curiosity, never meaning to use it.

The toxic contents of the plant root are vapourised by heat and diffuse into the local atmosphere.

However, he once explained to Mortimer what it was and what it was capable of, and he apparently stole some to murder his siblings, throwing it on the fire that evening just before he left.

Mortimer thought Sterndale would be at sea before news reached Plymouth.

Sterndale, of course, recognized the poison ’ s effects from the vicar ’ s description of the tragedy, and deduced right away what had happened.