‘What about chloroform?’ she suggested, though she knew she was getting desperate. ‘Could it produce stupor or delirium that lasts for days?’

‘No. Its danger is that it kills quickly. There have been fatal accidents due to its abuse: a druggist’s assistant in Aberdeen developed a habit of sniffing it from a towel at his place of work and was found dead face-down on the counter; and at Duke Street Hospital in Glasgow a doctor was found dead after inhaling it. But in both cases, it was obvious what had occurred.’

‘What if the victims were given it to drink? I know that Mrs Lyndsay recovered from drinking Dr Simpson’s chloroform champagne, but what if she had taken more? It’s relatively easy to get hold of,’ she added, anticipating Raven’s previous objection.

‘Unless you’re Dr Simpson,’ he muttered cryptically.

‘What?’

‘Never mind. Anyway, you would be able to smell it. Furthermore, it would be highly irritant to the mouth and throat unless very diluted. I observed no such marks on George Porteous, and if there were any on Mrs Johnstone, you can be damned sure Professor Miller and James Matthews Duncan would have seized upon it.’

Sarah frowned by way of admitting she was exhausted of suggestions, but Raven was not exhausted of arguments as to why she was wrong.

‘Even if the symptoms matched a known poison, what reason would a nurse have to murder her own patients? A private nurse, no less. She would be killing her employers and putting herself out of work. This is not like some scheming wife poisoning her rich husband so that she might live off the inheritance.’

Sarah could offer no response. The image rather uncomfortably made her think of Archie and his assurances that she would be provided for once he was gone. A hundred times a day she would enjoy the relief of forgetting, her mind focused upon something else, and then it would be dragged back. Remembering hurt every time.

‘I must be on my way,’ Raven announced, rolling down his sleeves and putting on his coat. He seemed content that the conversation was at an end, his case made irrefutably.

She would have to concede that she had no further arguments with which to present him. She remained suspicious of Mary Dempster and convinced that her sister Martha had not been entirely truthful with them. However, a sense or a feeling did not carry the weight of evidence, and nor could it make up for a lack of answers to the questions Raven had posed. Nonetheless, her instinct was to find the evidence rather than dismiss her theory.