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The Exchange Principle
(2025)(A book in the Margaret Harkness and Arthur Conan Doyle series)
A Story by Bradley Harper
Unravel a murder, uncover a legacy.
A scientist desperate to prove his theory… A journalist newly arrived to help him… A murderer unaware of ‘the exchange principle,’ where every contact leaves a trace…
Journalist and part-time investigator Margaret Harkness steps into the smoky, intrigue-laden world of 1912 Lyon, France, and witnesses the birth of forensic science when Edmond Locard, a man inspired by Sherlock Holmes, struggles to prove his ‘Exchange Principle.’ Every contact leaves a trace, he insists.
When a body is discovered in the opulent parlors lining Lyon’s back streets, Margaret and Locard employ nascent forensic methodology, striving to demonstrate the veracity and effectiveness of his Principle to skeptical judges and juries.
Can Locard and Margaret catch a killer by deciphering the vestiges of evidence left behind, or will this clever murderer slip through their fingers?
Based on true events and characters, The Exchange Principle is a story of unexpected connections and lasting impact in which the events of 1912 Lyon are told years later in post-war Florence, Italy. A final diary entry and an unexpected meeting reveal the imprints that Margaret Harkness herself will leave behind, a testament to the enduring power of human connection.
‘A tale of science, suspense, and the indelible marks we leave on each other.’
– Derek McFadden, author of What Death Taught Terrence
"A top of the table forensic mystery with a real life forensic detective!"
– Advance reader review
Margaret Harkness (1854–1923) was a trained nurse, journalist, and writer (as John Law) of several notable books, and was an independent woman who passionately advocated for the disadvantaged. ‘In her works of social investigation, Harkness uses a tone of social realism or naturalism, making her different from her male contemporaries.’ (Deborah Nord, Walking the Victorian Streets: Women, Representation, and the City. Cornell UP)
Enjoy more stories featuring this remarkable woman interwoven with real-life events by Bradley Harper.
A Knife in the Fog (novel)
London, 1888. Professor Joseph Bell and a young Arthur Conan Doyle join the hunt for the gruesome murderer soon to be known as ‘Jack the Ripper.’ Margaret Harkness, local to the East End streets of Whitechapel, might just prove to be an indispensable ally.
Queen’s Gambit (novel)
London, 1897. Margaret Harkness accepts a plea for help when her old friend and comrade, Professor Joseph Bell invites her to join him on a secretive mission. But she soon finds herself in an assassin’s crosshairs bent on two targets: both her and Queen Victoria.
The Exchange Principle (short story)
Lyon, 1912. Margaret Harkness steps into the smoky, intrigue-laden backstreets of Lyon, France and witnesses the birth of forensic science when Edmond Locard, a man inspired by Sherlock Holmes, struggles to prove his ‘Exchange Principle.’ Does every contact leave a trace? Or will a clever killer slip through their fingers?
Genre: Historical Mystery
A scientist desperate to prove his theory… A journalist newly arrived to help him… A murderer unaware of ‘the exchange principle,’ where every contact leaves a trace…
Journalist and part-time investigator Margaret Harkness steps into the smoky, intrigue-laden world of 1912 Lyon, France, and witnesses the birth of forensic science when Edmond Locard, a man inspired by Sherlock Holmes, struggles to prove his ‘Exchange Principle.’ Every contact leaves a trace, he insists.
When a body is discovered in the opulent parlors lining Lyon’s back streets, Margaret and Locard employ nascent forensic methodology, striving to demonstrate the veracity and effectiveness of his Principle to skeptical judges and juries.
Can Locard and Margaret catch a killer by deciphering the vestiges of evidence left behind, or will this clever murderer slip through their fingers?
Based on true events and characters, The Exchange Principle is a story of unexpected connections and lasting impact in which the events of 1912 Lyon are told years later in post-war Florence, Italy. A final diary entry and an unexpected meeting reveal the imprints that Margaret Harkness herself will leave behind, a testament to the enduring power of human connection.
‘A tale of science, suspense, and the indelible marks we leave on each other.’
– Derek McFadden, author of What Death Taught Terrence
"A top of the table forensic mystery with a real life forensic detective!"
– Advance reader review
Margaret Harkness (1854–1923) was a trained nurse, journalist, and writer (as John Law) of several notable books, and was an independent woman who passionately advocated for the disadvantaged. ‘In her works of social investigation, Harkness uses a tone of social realism or naturalism, making her different from her male contemporaries.’ (Deborah Nord, Walking the Victorian Streets: Women, Representation, and the City. Cornell UP)
Enjoy more stories featuring this remarkable woman interwoven with real-life events by Bradley Harper.
A Knife in the Fog (novel)
London, 1888. Professor Joseph Bell and a young Arthur Conan Doyle join the hunt for the gruesome murderer soon to be known as ‘Jack the Ripper.’ Margaret Harkness, local to the East End streets of Whitechapel, might just prove to be an indispensable ally.
Queen’s Gambit (novel)
London, 1897. Margaret Harkness accepts a plea for help when her old friend and comrade, Professor Joseph Bell invites her to join him on a secretive mission. But she soon finds herself in an assassin’s crosshairs bent on two targets: both her and Queen Victoria.
The Exchange Principle (short story)
Lyon, 1912. Margaret Harkness steps into the smoky, intrigue-laden backstreets of Lyon, France and witnesses the birth of forensic science when Edmond Locard, a man inspired by Sherlock Holmes, struggles to prove his ‘Exchange Principle.’ Does every contact leave a trace? Or will a clever killer slip through their fingers?
Genre: Historical Mystery
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