‘I saw her briefly when I first visited George Porteous,’ Raven replied. ‘A slight woman with dark hair. She sounded as if she was from Glasgow.’

He tried knocking a second time.

‘I don’t think anyone is home,’ he said.

‘I thought I saw someone at the window, but I might have been mistaken. Perhaps we should leave a message asking her to come to Queen Street, if indeed this is the right address.’

As she said this, Sarah heard movement from within. She and Raven shared a look, a moment of renewed optimism.

A moment later the door was opened by a bespectacled woman in a wool dress and knitted shawl, fair hair spilling from beneath a lace-trimmed cap. She was a little portly, an ample bosom filling out her dress.

‘I am sorry to keep you,’ she said. ‘I was not expecting visitors.’

‘Not at all,’ said Raven. ‘We are the ones who should apologise for calling on you unannounced. We might even have the wrong address. We are in search of a nurse by the name of Mary Dempster.’

The woman wore a regretful expression, shaking her head in a way that had Sarah preparing her weary feet for the walk to Charlotte Street in Leith and then all the way back to the New Town.

‘My sister is not at home,’ the woman said.

‘But this is her house?’ Raven inferred, optimism in his voice.

The woman’s brow furrowed in brief annoyance.

‘It is my house. I am Martha Dempster. Mary is my sister.’

‘I see,’ said Raven. ‘We were told a recent employer had procured Mary’s services by means of the Post Office directory, but the M. Dempster listed there is you.’

‘Indeed, though my sister does live here. Well …’

She seemed about to elaborate then changed tack.

‘Do you seek her for nursing work?’

‘No, though we do wish to speak with her concerning some of her previous patients. I am Dr Will Raven and this is my associate, Mrs Sarah Banks. May we come in?’

Miss Dempster seemed reluctant, fetching a glance behind her as though perhaps the house was in a state of disorder.

‘It is a matter of some importance,’ Sarah added.

‘Very well,’ she said. ‘But allow me a moment.’

She closed the door, leaving them on the threshold to ponder the delay.

‘Why would she keep us here?’ Raven asked, his impatience evident.

Sarah wanted to tell herself it was because he was desperate to save future patients from this new ailment, but she knew his motivations were less altruistic. Raven did not share all of the less admirable traits of his fellow professionals, but he was a medical man nonetheless, and he had the scent of glory in his nostrils.

She was more sanguine about the reason for the wait.

‘She is house-proud, I imagine, and we have caught her unawares. Tantamount to being intruded upon half-dressed.’