‘That this is what makes it such a dangerous new manner of disease. If it can spread so quickly to the leeches, then perhaps by investigating that, I might ascertain how it spreads to other people.’

Sarah could not believe she was having to spell it out. Raven was an otherwise rational and intelligent man. However, she understood what was impeding his thinking.

‘Do you recall what Martha Dempster said about what happens once an idea gets lodged in one’s head? You are so intent upon finding this new disease that you are missing what is obvious. There is a far simpler means by which Mary Dempster could be directly responsible for her patients becoming ever sicker and then dying. The leeches died because George Porteous had something in his blood that proved poisonous.’

‘Are you suggesting that the nurse administered something in error?’

The possibility of such a simple but deadly mistake had crossed Sarah’s mind, and she would have welcomed this less disturbing explanation. However, the numbers were against it.

‘One fatal misadministration would be unfortunate. Three suggests something less accidental.’

Raven took a moment to consider this, but that proved long enough to reject it.

‘I could say that your judgment is equally affected by a tenacious notion. Because of that horrible business last year, you are too easily inclined to see the possibility of poison.’

Sarah had to concede that if it was true of Raven, it could equally be true of her. Nonetheless, her theory chimed with the unease that had been in her mind since they visited that cottage at Lochend.

‘It struck me there was something awkward and oddly wary about Martha Dempster’s manner when we spoke to her, and it has taken me until now to deduce what that was. The woman was afraid of her sister.’

‘I got no such sense, and even if I had, it would be quite the leap to extrapolate that she must be a poisoner.’

‘Isn’t it suspicious that she cannot be found?’

‘We have barely looked. But even were we to entertain your notion for a moment, what poison would have produced such symptoms, leaving no sign of its administration?’

‘There must be a few possibilities.’

Raven fixed her with an unblinking look, like an impatient schoolmaster waiting for a pupil to admit she doesn’t have the answer.

‘Prussic acid?’ she suggested.

‘When it kills, it kills quickly and is not something easily procured,’ Raven responded dismissively.

Sarah ignored his tone. She was not yet ready to abandon her thesis and attempted to work through the problem logically.

‘Well, if procurement is an issue, the most commonly available household poison is arsenic.’

‘But arsenic produces symptoms of abdominal pain and vomiting, which were not observed in these cases.’

Sarah cursed inwardly. She ought to have remembered this. Nonetheless, there were still other possibilities.

‘Laudanum?’

‘The pupils would have been small. Pinpoint. They were normal in size.’