I counted down, and on zero Nightingale blew up the lab – at least, that’s what it felt like. A ball of burning fire, like a werelight spell gone horribly wrong, formed over Nightingale’s outstretched palm. A wave of heat washed over me and I smelled crisping hair. I almost threw myself behind a bench before I realised that the heat wasn’t physical. It couldn’t have been, or Nightingale would have caught fire. Somehow the heat was all contained within the sphere above his hand – what I’d felt was vestigia on a grand scale.

Nightingale looked at me and calmly raised an eyebrow. ‘How long do you want me to keep this up?’

‘I don’t know,’ I said. ‘How long can you keep it up?’

Nightingale laughed. I caught a flicker of movement in my peripheral vision and I turned to find Molly standing in the doorway, eyes shining with reflected fire and fixed on Nightingale.

I turned back just in time for the till to explode. The top blew right off and a spray of burning plastic fountained out, black smoke billowed upwards and raced across the ceiling. Molly gave a delighted shriek and I ran forward with the fire extinguisher and sprayed the CO2 over the till until it went out. Nightingale shut down his sphere of flaming death and switched on a set of extractor fans I didn’t even know the lab was equipped with.

‘Why did it explode?’ he asked.

‘The rapid breakdown of the components releases a volatile gas, hydrogen or something,’ I said. ‘I only got a C in chemistry, remember. The gas mixes with air inside the casing, there’s an electrical spark, and boom. The question I need you to answer is, does doing a spell suck magic out of an object or put magic into an object?’

The answer was, of course, both.

‘You don’t normally cover this until you’ve mastered the primary forma,’ said Nightingale. Magic, as Nightingale understood it, was generated by life. A wizard could draw on his own magic, or on magic that he’d stored by enchantment, which sounded interesting but not relevant to exploding cash tills. However, life protected itself, and the more complex it was the more magic it produced, but the harder it was to draw off. ‘It’s impossible to draw on magic from another human being,’ said Nightingale. ‘Or even a dog, for that matter.’

‘The vampires,’ I said. ‘They sucked the life out of everything in the house, didn’t they?’

‘The vampires are obviously parasitical in that way but we don’t know how they do it,’ said Nightingale. ‘Nor do we know how people like your friend Beverley Brook draw power from their environment, either.’

‘The vampire house is where I first noticed the effect on the microchips,’ I said.