Barak looked impressed. 'All these years I've wondered what it was for. You are a knowledgeable man, apothecary. Ah, that stings!'

Guy dressed his burn, coating it with a harsh-smelling oil, then sent him back to his room while he dressed my arm. I winced as he lifted my sleeve to expose the livid red mark, the puckered skin. He applied some of his oil and I felt the smarting ease a little.

'What is that stuff?'

'Oil of lavender. It has cold and wet properties, it draws the dry heat from the fire that has stung your flesh.'

'I remember you using it on the young founder who burned himself.' I looked at him seriously. 'There is a fire I think no amount of lavender could quench. Guy, I was going to talk to you anyway, ask you some questions about the matter that has caused all this death and ruin. It involves alchemy, as I told you, and there are aspects that have me sore puzzled. I would tell you all, if you will listen.'

'Is it safe for me to know?'

'If you keep it close, there should be no danger from those that pursue us. But I will not tell you if you would rather not know.'

'Cromwell would not be pleased, I think. I note you have waited till friend Barak was gone.'

'I'll take the risk if you will.'

'Very well.'

As he bound my arm with a strip of cloth, I told him all I knew of Greek Fire, from Cromwell's first summons to the fire last night. As he listened his face grew more troubled.

'Your aim is to catch these killers?' he asked.

'Yes. They have killed five people now. The Gristwood brothers, Bathsheba and her brother, and the watchman. A founder called Leighton is probably dead too.'

'I remember you were asking about founders.'

'Yes. I think we were too late to save him. And there are three more people kept in hiding for fear of these monstrous rogues. I want to catch them, stop them cutting this swathe through London.'

'And to recover the formula for Greek Fire for Cromwell?'

I hesitated. 'Yes.'

'Have you considered the havoc such a weapon may wreak? It could burn whole navies. It could be used to fire a city, as we saw last night.'

'I know,' I said quietly. 'The image of great ships on fire forever comes unbidden to my head. But, Guy, if Cromwell does not get it, others will, foreign powers who would use it against England.'

'And take her back to Rome?' He raised his eyebrows, and I remembered he was neither English nor Protestant. He considered for a moment. 'What did you wish to ask me?'

'Do not answer if you feel you cannot. But I know now that there was a barrel of Greek Fire kept at Barry's for a hundred years. And that there was a formula. My belief is the Gristwoods used the period between their discovery last October and their approach to Cromwell in March to build their apparatus – there is evidence of that – but also to try and make some more, using the formula.'

'A barrel would not last for ever.'

'Exactly. And with two ships destroyed, most of the barrel is probably used up. That they could set that fire last night may indicate they have made more. But how, Guy? How does an alchemist create material from a formula?'

'By finding the correct mixture of the four elements. Earth, air, fire and water.'

'That all things are composed of. Yes, but that is no easy task.'

'To be sure. It is easy enough to make iron, using the minerals God has seeded in the earth, but hard to make gold or we would all be eating from gold plates and the stuff would be worthless.'

'And to make Greek Fire, how easy might that be?'

'Without the formula, it is impossible to tell.'

I sat up. 'You spoke of iron and gold just then. There are some things that are common, easy to find, like iron, and others like gold which are very rare.'

'Of course. That is obvious.'

'I have been reading about the history of fire weapons in the east. We know the Byzantines had no problem in finding the elements needed to create the liquid that is set on fire. Equally, similar substances are mentioned by the Romans, but they were not developed as weapons. I think that perhaps a crucial element needed to manufacture Greek Fire is hard to get. I think the Gristwoods may have been looking for a substitute for this missing element. This could have led them to the Polish drink that burned the table at the inn.'

He stroked his chin. 'So they used that to make Greek Fire?'

'I don't know. Perhaps.'

'And, from what you say, they were already working with the rogues who were to become their killers in a plot against Cromwell?'

'Yes. I don't know how that came about. But, Guy, if I was able to find some of the original Greek Fire in the Barty's churchyard I told you of-'

His face wrinkled in distaste. 'Desecrating graves-'

'Yes, yes, I agree. But it is to be done anyway. If I found some and brought it to you, could you analyse it for me, distil its essence or whatever it is you do?'

'I am an apothecary, not an alchemist.'

'You know as much of their arts as most of them do.'

He took a deep breath and folded his arms. 'To what end, Matthew?' he asked.

'To help me find out what has happened-'

He broke in sharply. 'Matthew, you forget what you are asking me. To analyse Greek Fire so that Thomas Cromwell may have the secret.' He paced the room, his dark face more serious than I had ever seen it. At length he turned to me.

'If you can find this cursed stuff and bring it to me I will look at it. But then I will destroy it. I will give you no clue to its manufacture that may help Cromwell. If my researches throw up anything that will aid you to catch these murderers without doing that, I will tell you. I am sorry, Matthew, but that is all I am prepared to do.'

'Very well. I agree.' I extended my hand and he took it. He still looked serious. 'St Gregory of Nyssa once said all the arts and sciences have their roots in the struggle against death. And so they should have. This thing of ruin and destruction is a perversion, a monstrosity. If you find that formula, you should destroy it and all the world will be safer.'

I sighed. 'I am bound to Cromwell. And to help my country.'

'And how do you think Cromwell, and King Henry, would use Greek Fire, ruthless men of blood that they are? For murder and mayhem, that is how.' He was angry. 'This is far worse than Scarnsea, Matthew. Cromwell has used you again not just to hunt a murderer, but to aid him in a brutal, cruel blasphemy.'

I bit my lip.

'And Barak,' he continued, 'how does he see things?'

'He is utterly loyal to his master.' I looked at Guy. 'I will tell him nothing of this conversation.' I leaned back on the bed with a sigh. 'You do well to upbraid me,' I said quietly. 'I have worried about what Greek Fire may do, but – yes, I have been driven on by a passion to catch these murderers, recover what was stolen. And to save Elizabeth Wentworth. At any cost.'

'That cost may be too high. You must decide when the time comes, Matthew. It will be between you and God.'