‘Were there other symptoms? What of her temperature, her heart rate?’ Raven asked.

‘There was no fever. The pulse was irregular at times. All quite consistent with my diagnosis.’ Fowler shook his head and smiled at Raven, suddenly more at ease. ‘I see no evidence of contagion here. If there was, the nurse would surely be affected, as she spent more time with either of them than I did, or Miss Porteous.’

Raven straightened in his chair.

‘Of course. The nurse. We should establish whether she is suffering any of the same symptoms. Do you know who she was?’

‘Yes,’ Dr Fowler replied. ‘I have recommended her to several patients before. Her name is Mary Dempster.’

Raven looked astonished.

‘Did you say Mary Dempster?’

‘Yes. What of it?’

‘That is the name of the nurse who treated Mrs Johnstone.’

Dr Fowler’s expression quickly progressed from smugness to confusion as the significance of the exchange was lost on him.

‘Three of her patients have died exhibiting similar symptoms,’ Raven went on.

‘Then she is in danger,’ said Sarah. ‘She might already be ill.’

Dr Fowler looked at her as though she had just materialised in front of him, like one of the spectres he claimed had tormented Mrs Porteous.

‘Or perhaps she is merely serving as a conduit,’ Raven suggested. ‘The unhappy medium carrying the contagion of disease from one patient to another.’

‘Like puerperal fever?’ Sarah asked.

‘Exactly!’ Raven said, delighted she was following his logic.

‘This is nothing like puerperal fever,’ Dr Fowler protested.

‘It is not the disease but the contagious communicability that might be the same,’ Raven insisted.

Fowler slumped in his chair, completely at a loss.

‘Where does she live?’ Raven asked. ‘Do you have her address?’

‘Not to hand,’ he said, getting to his feet. He wandered towards the door, leaving Sarah unconvinced he would remember what he had gone in search of by the time he reached the hall.

‘The nurse is the key to this,’ Raven said.

‘What makes you so sure?’

Sarah was concerned that his enthusiasm was getting the better of him, clouding his judgment in much the same way that he himself suggested the professor’s had been compromised with respect to chloroform.

‘Simpson has written about this very thing, in his paper comparing puerperal and surgical fever. In 1840 in Manchester, four hundred women in different parts of the town were delivered by twelve midwives. Sixteen died of puerperal fever, all sixteen having been delivered by the same midwife. There must have been something connected to that one midwife which meant that all of her patients took the disease while the patients delivered by the other midwives escaped it. Some form of morbific principle attached to her hands.’

Sarah opened her mouth to tell him she had read this very paper but Raven was in full flow.