As the blank slid free of her fingers, something followed it – a kind of string, was how he thought of it, except that it was a string of strange, pulsing light. Almost like a static shock, but too delicate, too lingering.

A string of orange light that broke just as the blank thumped to the carpeted floor.

She didn’t say a word. Her eyes had gone wide, but then they narrowed in calculation, and she backed slowly away.

He staggered and braced himself against the doorway. Just breathed for a moment, and then reached over and closed the common room door. Then he locked it and pocketed the key.

‘What was that?’ he asked, and when she didn’t answer, he pushed free and stepped forward. She backed up. ‘You’re not leaving until you tell me.’

‘I don’t know what happened,’ she said. He could see her trembling. ‘The blank must be—’

‘Don’t try it. The blank isn’t defective, and I’m not a fool.’

‘Jess—’

‘I can only think of one explanation for what I just saw, and that is that you’re an Obscurist,’ he said.

‘I’m not!’

‘Don’t lie to me again.’

He saw her whole body go tense and still. She was considering whether or not to come at him for the key, and whether or not she’d win if she picked that battle. It lasted a long few seconds before she took in a breath and said, simply, ‘Yes.’

Now that she’d admitted it, the shock rolled over him. Obscurist. But they weren’t supposed to ever leave the confines of the Iron Tower. What was someone like that doing here, disguised as a student?

Maybe that’s the point. Maybe it’s another test, and we’re supposed to find her out. ‘Does Wolfe know what you are?’

She snapped the answer back too quickly. ‘He doesn’t know anything.’

‘Bit of advice, if you’re going to lie, learn to do it better.’ Jess’s pulse was racing, but it was as much with adrenaline as fear. I’ve seen an Obscurist at work. That seemed as impossible as petting a unicorn. ‘Relax. I won’t hurt you.’

That made her frown, and her voice turned firmer. ‘Do you realise how arrogant that makes you sound? If I’m an Obscurist, do you really think you have the ability to hurt me?’

‘Probably,’ he said. ‘They don’t keep you in the Iron Tower because you can easily defend yourselves, now, do they? You’re not some sorcerer out of a story. What you do is alchemy, not magic. You’re not going to throw a spell at me. Alchemy requires preparation.’

‘I wasn’t talking about magic,’ Morgan said. ‘I can look after myself. And, if you push me, I will.’ She had a knife now, and he hadn’t even seen her draw it. From the way she held it, he could see she was comfortable with the weapon … and she would be, if she’d actually survived a war to get here.

But there really would be no advantage in fighting, for either of them. He held up his hands. ‘Good point. Maybe I should just call the High Garda and have you escorted to the Iron Tower.’

He’d hit a nerve. A big one. She took a tighter grip on the knife, and he saw the flash of panic in her eyes. She didn’t want to go there. Not at all.

‘All right,’ she said, and tried to make it sound casual. ‘Wolfe knows all about me. Happy now?’ He might not have believed her if he hadn’t just seen her lie, but that, surprisingly, was the truth. Though why Wolfe would help an Obscurist was another thing entirely.

‘What are you going to do?’ she demanded.

‘I don’t know.’ He nudged the blank on the floor with his foot, but it was back to just a plain volume, no different than any other he’d ever held. ‘Is this thing dangerous?’

‘It’s a blank. Why would it be dangerous?’

‘Because I just saw it do something I’ve never seen a blank do before.’

‘That’s not the book,’ Morgan said. ‘It’s just simple manipulation of the formulae behind the mirroring. I can do that with any blank. They’re all connected to the Codex, by their nature; it’s the principle of similarity. As above, so below. It’s what the Doctrine of Mirroring is based on. I was finding a way in.’

She said it as if that was self-explanatory, which maybe it was, to her; it was the same offhand way Thomas talked about engineering, or Khalila about dizzying levels of mathematics, as if anyone ought to be able to see how it worked.

It made him feel stupid, and annoyed by it. ‘So you’re an Obscurist who came here to pretend to be one of us,’ he said. ‘Why? Is this another one of Wolfe’s bloody stupid tests? Are we supposed to discover your secret? Then I think I win. Though it was stupid of you to be down here doing this.’

‘It’s not a test! I wish it was. That would be so … simple.’ The flush was fading from her cheeks now, and she walked over to the fire to warm her hands. ‘And I didn’t do it here by choice. The blanks work best when they are near each other. Principles of similarity, the sympathetic energy grows stronger. I locked the door. What are you doing here?’

‘Looking for sleep,’ he said. ‘Which I’m not going to get. If you’re not here to test us, why are you here? Shouldn’t you be in the Tower?’

‘I’m not going to the Iron Tower,’ she said, very quietly. ‘That’s the whole point of this. They were looking all over England for me by the time I made it past the border. I won’t be here long. Once I have what I came for, I’ll be on my way again.’

‘Khalila was right. She told me not to trust you,’ Jess said. He sat down on the divan, because he didn’t think he had the strength to keep standing; too many surprises today, and not enough rest. ‘What are you after?’

‘What do you think? I want my life! I want to erase any trace of … what I am.’ She wrapped her arms around her body, as if she was chilled to the bone, despite the fire. ‘I was coming here, you know, that wasn’t a lie. I’d already been accepted for training when I first accidentally opened up formulae; Scholar Tyler in Oxford saw me do it when I was reading a blank at the Serapeum. He told me opening the formulae leaves a kind of … record that the Obscurists could trace back to me, eventually. I had to destroy my record in the Codex itself if I didn’t want to end up in the Iron Tower.’

She paused, but Jess didn’t say anything. Her voice had the ring of truth. More, it had the ring of desperation.

‘I could open formulae, but actually altering it was impossible to do from Oxford, and even from the London Serapeum; I tried. Scholar Tyler told me that the closer I could get to the Iron Tower, the better chance I had of changing it. I already had an opening here in the training class. It was my only choice, they were looking for a stray Obscurist in London by the time I left.’ That struck some kind of thought in her, and she looked at him with sudden, real distrust. ‘Did someone send you here to find me? Did you suspect me?’

‘Not me. I was just looking for a quiet place to kip. You should have put a sign up. No entrance, alchemical sabotage in progress.’

‘Was that a joke?’

‘Not a very funny one.’ Jess still couldn’t quite take it in. An Obscurist. He’d come to think they weren’t real, or if they were, that they were incredibly old, with beards that stretched to the floor. He’d never imagined one his own age. Or a girl, for that matter. ‘You said Wolfe knows. How did he find out?’

‘He caught me,’ she said. ‘I tried my best, but if I’m not concentrating sometimes I reveal the formulae without meaning to do it, and … he saw. I thought he’d send me straight to the Tower. Instead, he told me to do what I needed and get out as soon as possible. He warned me my time was running out, and he couldn’t protect me.’

The idea of Wolfe protecting any of them made Jess feel oddly off balance. Wolfe was their enemy – or, at least, their judge, jury, and executioner. What would move him to keep Morgan’s secret?

He didn’t think she knew, or if she did, that she’d tell him. ‘If you came just to remove this record from the Codex, it means you won’t be staying once you do it. Right?’