‘Citoyen Bisset.’ She put coins on the counter to pay for the map.

Bisset made a show of writing up the sale in his ledger, then handed over the map along with a furl of assignat notes. Ada peeled off one note and gave it back to him.

‘For your trouble.’

‘Many thanks. Your father apologises for not sending so much this time. He also left a message.’

He pulled a folded and sealed letter from a stack of papers. Ada saw her father’s fluid, looping hand on the front. Her heart stuttered. For a moment she considered ripping up the letter and leaving that as her return message.

But she knew she wasn’t that person. She stuffed the letter into her pocket along with the notes. Without a word, she hurried out of the shop and steered James back towards the river. She wanted to get as far away from her guilt as possible.

James spied the book under her arm.

‘Is that Galvani’s Commentary? Mind if I borrow it when you’re done?’

Ada looked at him in surprise. ‘You’re interested in electricity?’

‘I dabble. I’m a first year in medicine at St Bart’s – the London Hospital Medical College, if you want to be formal about it. I’m curious about the medical applications of electricity.’

‘You’re not at Oxford?’ asked Ada, changing the subject.

James shook his head. ‘No chance. They’re still stuck in 1300 and think modern medicine is some sort of devilry. Not a decent medical school to be found outside the capital – unless you go all the way up to Edinburgh, but my mother wasn’t keen on me going so far.’

‘You can read it when I’ve finished, if you like.’

He turned his dazzling smile on her. ‘Thanks.’

As they made their way back, their conversation shifted from trading favourite periodicals to competing theories of electricity.

‘Galvani’s got it right, I think,’ he said. ‘Electricity is something native to the body. It must be. Look at its vitality, its force, its dynamism. If it’s not discovered to be some facet of the soul, I’d be astounded.’

‘You still believe in the soul?’

‘Of course. I see it in the very being of each person I meet.’

She shook her head.

He smiled. ‘You think me foolish?’

‘No. Just naive. The soul died long before the guillotine arrived in Paris.’ She looked away, at the scant supplies in the shops and the hungry people queuing, dressed in scraps and rags. ‘It died when rich men in charge forgot we’re human too.’

She saw his smile falter.

A mail carriage halted them at the edge of the Rue St Denis, rumbling past with its six-strong dray horses clattering over the cobbles and metre-high wheels kicking up mud. When it passed, the silence between them fell heavy.

‘In England, do women participate in scientific research?’ she asked eventually.