- Hewson entered the scene of anatomy at a really ideal crossroads.

This was the time that they were looking at practical dissections as an art of teaching.

And instead of just researching these things and reading about them, there was much more of a hands-on approach to medicine and science.

- [Narrator:] So did the activities of William Hewson hold the key to the gruesome discovery at Craven Street?

Perhaps the bones were not the result of murders, but were, in fact, anatomy specimens.

The pit of bones held a vital clue.

Hewson conducted extensive research into the human lymphatic system.

And his detailed findings were recorded by leading anatomical artists of the day.

But to earn his place in the Charter Book, he'd gone a step further, giving a groundbreaking lecture at the Royal Society.

- Now, up until this point, it was believed that humans had a lymphatic system.

But Hewson really set out to prove that this existed in other species, as well.

- [Narrator:] His test subject took everyone by surprise.

It wasn't human at all -- it was a green sea turtle.

- Now one of his most ingenious experiments took place with a dead turtle.

And Hewson basically injected mercury into the turtle and watched how it went through the lymphatic system.

- [Narrator:] Could the turtle in Hewson's experiment be the very same one Professor Hillson discovered in the pit of bones?

- This is some of the bones of the shell of the turtle.

And one of the fascinating things was there was a little bead of mercury actually resting inside the bone of th shell.

And it's exciting because it's a very clear association with Hewson.

The find of mercury in association with the bones of a turtle.

- [Narrator:] This was a dramatic turning point in Professor Hillson's mission to find the source of the bones at Craven Street.

He now had compelling proof they were the result of William Hewson's work.

Could Benjamin Franklin be ruled out as a suspect?