'You seem strung tight as a bowstring, Matthew.'

I sat down, rubbing sweat from my brow. 'Not surprising, as someone has just tried to kill me.'

'What?'

'You'd get it out of me in the end. I can't tell you all, but Lord Cromwell has spared Elizabeth Wentworth from the press for two weeks provided I undertake a mission for him. Nothing to do with the monasteries this time, but murder again and roguery-' I broke off, looking through the window. 'Young Barak, who I see tying his horse up outside, has been deputed by Cromwell to assist me.'

'Then the help you want is for Cromwell?' Guy looked at me seriously.

'To help to catch a brutal killer. I am not allowed to say more – I should not even have mentioned Cromwell's name. It is too dangerous.' I sighed. 'I will not press you if you feel you cannot in conscience help.'

The door opened and Barak came in. He looked uneasily at the bottles and jars lining the walls, then at Guy with his dark face and apothecary's robes. Guy bowed.

'Master Barak, I pray you are well.' He mispronounced the W in his lisping accent as he always did. I realized how strange and foreign he must seem to Barak.

'Thank you, master apothecary.' Barak stared around. I guessed he had never been in an apothecary's shop before; he looked like he had always known rude health.

'Would you like a little beer?' Guy asked him.

'Thank you,' Barak answered. 'It is a hot day.'

Guy went out to fetch it and Barak came over to me. 'The earl's worried. He's had Kytchyn taken to a place of safety till this is over.'

'Thank God for that.'

'He says you're being too slow. He's concerned you won't be seeing Lady Honor till tomorrow. There are only ten days till the demonstration, and the king's told him he's looking forward to it.'

'Perhaps he should seek a miracle worker then.'

Barak stepped away as Guy returned, bearing two cups of small beer. I drank gratefully, for I was very thirsty. Guy stood at the end of his table and studied Barak carefully a moment; I was pleased to see Barak look uncomfortable under that penetrating gaze.

'Well, then,' Guy said quietly. 'What help do you both wish from me?'

'We have to deal with alchemists,' I said. 'I know nothing of their trade, and would welcome your advice.' I opened the satchel and laid the alchemical books on the table. Then I carefully took the bottle from my pocket and held it out. 'Have you any idea what this strange stuff might be?'

He opened it carefully, then poured some on his finger and sniffed. 'Be careful, it burns like fire,' I warned as he bent and touched his tongue to it.

To my surprise, he laughed. 'There's nothing to worry about,' he said. 'There's no mystery here. This is aqua vitae, though distilled to a very high concentration.'

'Aqua vitae?' I laughed with astonishment. 'This new stuff that is distilled from bad wine and prescribed for sore eyes and melancholia?'

'The same. I think its value overrated, it just makes people drunk.' He rubbed the stuff between his fingers. 'A cupful, they say, will blind a horse. Where did you get it?'

'On the floor of an alchemist's workshop that had been – abandoned.' He looked at me sharply.

'Never mind where we got it, apothecary,' Barak cut in. 'Are you sure that's what it is?'

Guy gave him a long look, and I feared he would order him from his shop, but he turned to me with a smile. 'I believe so. Though the thickness of the liquid and the fiery taste suggest the concentration is very strong. I believe I may even be able to tell you where it came from. But first, there is a way of proving what it is. I will show you. It is quite spectacular, Master Barak. Wait a moment.'

He put the bottle down carefully, then left the room.

'Listen to me, Barak,' I said. 'Guy is a friend: have a care how you speak to him. And he is not one to be bullied like that doorkeeper. You will only anger him.'

'I don't trust him, on his looks.'

'I think that's mutual.'

Guy returned, carrying a candle and a small glazed dish. He closed the shutters, then carefully tipped a little of the liquid into the dish. Then he touched the candle to it.

I gasped, and Barak stepped back, as a blue flame flared in the bowl, rising two inches into the air.

'You'll burn the shop down!' Barak exclaimed. Guy only laughed again.

'The flame is too weak to set anything alight and it will die in a moment.' Sure enough, as we watched, the blue flame sank as quickly as it had risen, turned yellow, guttered and went out. Guy smiled at us. 'There. That is a characteristic of aqua vitae, that blue flame. It was certainly a very strong mixture.' He opened the shutters again. 'Note there is no smell or smoke.'

'You said you might know where it came from,' Barak said, his tone more respectful now.

'I did. We apothecaries are ever on the lookout for new herbs, new concoctions, from the strange parts of the world Englishmen voyage to nowadays. It is the constant topic at the Apothecaries' Hall. A few months ago we heard of a cargo that had been landed at Billingsgate from a ship that had ventured into the Baltic trade, to the lands of endless snow. They brought back a cargo of a colourless liquid they say men drink there. When people tried quaffing it here, as they would beer, it made them very sick. This sounds like the stuff.'

'What happened to the cargo?'

'That I do not know. I think one or two of my brethren went after it as a curiosity, but were told it had been sold. You would need to enquire among the sailors' taverns to find more.'

I nodded thoughtfully. A thick, viscous liquid that burned in a strange way. In some ways it sounded like Greek Fire, but in others quite unlike. The liquid in the monastery had been black, with a strong smell, Kytchyn said, and the flame we had just seen could never have set light to a ship. But what if this stuff was part of the formula, what if it changed its behaviour if other things were added?

'What do you know of alchemy, Guy?' I asked. I took the alchemy books from my satchel and laid them on the table. 'These books are so full of mysteries and jargon I can scarce understand a word.'

He picked one up and leafed through it. 'Alchemy has given itself a bad name. Perhaps worse than it deserves. The alchemists like to keep their trade cloaked in secrecy and fill their books with references only they can understand.' He laughed. 'Some of the old books I think nobody understands.'

'And it impresses people, makes them think there must be a great mystery there to be uncovered.'

Guy nodded. 'But in that they are no worse than some physicians with their ancient remedies and secret formulae. Or lawyers, for that matter: in some courts you plead in old French no ordinary mortal could comprehend.'

There was a bark of laughter from Barak. 'He has you there.'

Guy raised a hand. 'And yet alchemy is part of natural science, the study of the world around us. God has left signs and clues in the world, that by struggling we might come to understand things: cure diseases, grow better crops-'

'Turn lead into gold?' I hesitated. 'Set water on fire?'

'Perhaps. And the task of alchemy, like astrology and medicine for that matter, is to read those clues.'

'As rhinoceros horn is supposed to bring virility, the clue being its resemblance to the male organ. But, Guy, so much of this looking for signatures and correspondences is mere fraud.'

'Yes, it is. I agree that the manner in which alchemists profess secret, arcane knowledge is often no more than a trick to keep their trade inaccessible.'

'So you think, like most, that alchemy's a suspect trade?'

'Not altogether. There are plenty of rogues who claim to have found the philosopher's stone that can turn base metal into gold, but for each one of them there is another who has striven to make real achievements by careful observation, by study of how substances are made up and how they change. How the four elements of earth, air, fire and water interact to make all the things we know. How heat can change one thing into another – wine into aqua vitae, for example.