‘And how do we do that?’ Raven’s voice had grown louder in his frustration. Sarah put a finger to her lips. This was not a conversation that either of them wished to be overheard.

‘It is unfortunate that there have been no post-mortem examinations carried out,’ Sarah observed. Raven wondered if this was a veiled criticism of himself: he had not pushed for one in the Porteous case, though convincing Dr Fowler of its necessity would have been difficult.

‘Even if there had been post-mortems, they might not have shown anything,’ he replied. ‘In order to detect poison in a corpse, we need to know what poison to look for.’ Back to that again. They seemed to be going around in circles.

‘We should speak to Dr Fowler again,’ Sarah suggested. ‘Find out who Mary has worked for in the past and what happened to them. If he was Mrs Eddlestone’s physician, perhaps he can shed some light on what happened at Trinity: what the cause of death was in each case and whether he prescribed the morphine Mary was seen disposing of.’

‘I would be wary of giving too much credence to his answers,’ Raven warned. ‘But even though she was caught disposing of it at the Trinity house, I doubt that morphine was her modus operandi. It does not match the other patients’ symptoms, and its effects would be too obvious, even to an old fool like Fowler. I think that our Nurse Dempster is far more cunning than that.’

Sarah’s face suddenly contorted. She bent over and grasped at her side.

‘Sarah, what is it? Are you unwell?’

She took a couple of deep breaths and straightened up again.

‘It’s nothing. Just a twinge.’

‘Has this happened before?’

She looked at him. Paused for just a little too long.

‘Once before,’ she admitted.

‘I should examine you,’ Raven said.

‘There is no need.’

‘How far along are you?’

She gaped at him in surprise, which he hoped did not betray a lack of confidence in his abilities as an obstetrician. She looked as if she were about to deny it but seemed to change her mind.

‘I haven’t bled for two months now. There is no need to worry about me, though. I am quite well.’

Raven hoped that this was the case but resolved to look again at the obstetric textbooks he had been studying in the wake of the Glassford case.

‘At least come and sit for a moment,’ he said, leading her to his consulting room.

He sat her in a chair and closed the door.

‘I am wary of Quinton seeing us together again. He seems to miss nothing that goes on under this roof. He has even taken to monitoring our use of medical supplies. Counts out drops of chloroform as if they were gold coins.’

‘He is not so all-knowing,’ Sarah stated emphatically. ‘He has yet to find his thief.’