‘Do you still entertain any doubt that she killed those people?’

‘None,’ he replied. ‘In fact, I believe that she might also have killed her stepmother, her stepfather and her sister’s fiancé.’

Sarah’s face, already pale, blanched further.

‘What makes you think that?’

‘They all died quickly, unexpectedly, while under the same roof as Mary. The stepmother, who Mary detested, died last. A perfect precursor of what happened at Trinity.’

‘You suspect her motive to be what? Vengeance?’

‘She was treated cruelly by her stepmother. She killed those the stepmother loved, and then the woman herself.’

‘But why would she then go on to kill her patients? What motive would she have for that? And why did she spare Martha?’

Raven admitted he’d been asking himself the same question.

‘I think that you were right when you suggested Martha was afraid of her sister, but there is undoubtedly a bond between them. I suspect Martha is useful to Mary in some way, but I cannot fathom precisely how.’

‘I do not think that bond would survive were Martha to learn that Mary had murdered three of the people she loved. Perhaps if we apprised her of our suspicions, she would be more forthcoming.’

‘That occurred to me, but I did not feel it was the time to tell her. Nor did it seem likely she would be convinced.’

‘Certainly not without better evidence,’ Sarah said. ‘The truth of it seems plain to us, but we remain without solid proof that Mary has poisoned anyone.’ She began to button up her coat. ‘And if we are to take our suspicions to the authorities, we need to build a case that would merit investigation. We need more than supposition and coincidence. People die all the time. How can we prove that she is killing them?’

Raven had been grappling with the same question. His proposed motive of vengeance would not explain all the deaths that had come to their notice, and he had no idea what method she was using to achieve her nefarious ends. She had presumably killed Stuart Eddlestone with morphine, but that did not match the symptoms of her other victims.

‘We might never know how many have died at her hands,’ Raven said, giving voice to nothing more constructive than his vexation. He paused for a moment, his brow creased in thought. ‘What do we know for sure?’

‘We know that people die in numbers when Mary is around, and the death of Dr Fowler’s leeches is suggestive of mischief,’ Sarah replied. ‘According to Christison, there have been reports of leeches dying when they were applied in cases of poisoning with oxalic acid and opium.’

‘But our cases do not fit with either.’

‘We need to find out what she is giving them.’