Simon has returned to them.

His shoulders stoop low. He no longer wears a coat, and his shirtsleeves have been rolled back to the elbows. Carbolic soap has scrubbed his hands and arms clean, but Agnes knows exactly what they have been doing.

Her chest turns over.

She keeps her head bent while Simon exchanges a few words with the clergyman and takes a seat by her side. She cannot bear to think of his hands moving over Pearl, or of what substances lay embedded beneath his fingernails.

‘Miss Darken,’ he says softly.

‘Is it done?’

She feels, rather than sees him nod.

‘And do you know … ?’

Simon exhales heavily. ‘The coroner found the cause of death I was anticipating. Miss Meers died of phosphorus poisoning.’

It makes little sense – yet what has of late? She is no doctor. For all she knows, being confined in that house with Mr Meers might have caused the infection to leach through into Pearl, like tea leaves colouring water.

‘I suppose the miasma …’ she begins dully.

‘No,’ Simon replies. He takes a moment, seems to be grappling with some powerful emotion. ‘The phosphorus entered the body through the stomach.’

The stomach. Like that suicide weeks ago, the man who ate matches. ‘But how—’

‘I do not like to tell you this, Miss Darken. It will distress you, but I would rather you hear it from me than another source. You mentioned a glow around the young girl. An – what word do those numbskulls give it? – ectoplasm?’

She swallows. ‘Yes.’

‘That glow was manufactured. Miss West achieved it, it appears, by crushing match heads and concealing them in jam to mask their unpleasant taste.’ He takes a breath. ‘She then fed the poisoned jam to her sister. Whole jars of the contaminated substance were found at the property on Walcot Street.’

Agnes sways.

Miss West killed her sister.

Poisoned her.

Can such wickedness exist in the world?

When Agnes thinks back, the clues were all there: Miss West’s frustration and aversion to the obligations thrust upon her; her preoccupation with money; Pearl’s strange, sickly glow. If she was clever like Simon, she would have realised.

She could have helped.

But as with her own sister, she did not spot the signs until it was too late.

‘How could she do it?’ she gasps. ‘After seeing what phosphorus did to her stepfather … How could Miss West feed her little sister matches?’

Simon places a hand on her shoulder, holds her steady. ‘Miss West proved to be a very bitter young woman indeed.’

Footsteps sound on the tiles. Sergeant Redmayne paces towards them. ‘Is she …’

Simon assesses Agnes and nods to the policeman without saying a word.