So I had to wait, and become a proper tourist for once, and indulge my ever-growing passion. Four more days, in fact, before the letter finally reached me – wonderful days, spent in the autumnal warmth, and often enough with Louise, for the more rendezvous I had with her the more I wanted. After the events at the Marchesa's salon, we threw off all caution and discretion. I began buying her presents, we walked together in the city, were seen together. It made me proud and uncomfortable at the same time – I once had to tell her to be more discreet with her husband.

'I will leave him now, because of you. Now I know what it is really to love someone, I cannot stay any more. We can be together for ever, then,' she said, turning to look me in the eyes. 'We can be like this forever. Just you and me.'

'What about your son?'

She made a gesture of disgust. 'He can have him. He is not my child; I merely bore him. There is nothing of me in him at all. He will be like his father; weak, useless.'

'He's only four.' She had spoken with a harshness I had never heard in her before. There was real cruelty in her words and they disturbed me.

I must have reacted, for she instantly changed. 'Oh, I love him, of course I do. But I am no good for him. I don't understand him.'

Then she put her arms around me once more, and changed the subject totally for an hour. But I left our rooms with an uneasy feeling that afternoon; it faded soon enough, but did not disappear entirely.

It also changed the way we were together; Louise did not refer to leaving her husband again, but more and more often the conversation came round to her desire to be with me. I could understand why her life was hellish, and why she so desperately sought a means of escaping. When I considered the weals and cuts, Cort's behaviour at the séance, his hallucinations, the indignities and humiliations she endured when no one was there to see, it was hardly surprising she clung to me.

And I was besotted with her. So why did I not leap at the chance to capture her forever? It could have been done. A separation of some sort from my wife was possible, messy and unpleasant though it might be. But Louise and Venice were linked too closely together. Love and city were intermingled; I could not imagine one without the other, and I think my hesitations and doubts were linked to my sluggish awareness of my growing immobility. The Marchesa was right; Venice was like an octopus, which slowly and stealthily entangled the unwary in its tentacles until it was too late. Longman would never leave; Cort might not either; in other Englishmen I met in that period I learned to recognise the slightly vacant look of the entranced, the people hypnotised by the light, who had lost their will-power, voluntarily given it up, like the followers of Odysseus on the island of the lotus-eaters.

They did not enter a state of bliss by so doing; Venice does not offer happiness in exchange for servitude. The opposite, rather: melancholy and sadness are its gifts; it allows the sufferers to be all too aware of their lassitude and inability to leave. It taunts them with their weakness, but still will not let them go.

Some were immune; Drennan seemed unaffected, for example. Nor did it have any effect on Macintyre, because he scarcely knew where he was. For him Venice was merely the place where his workshop was; he had sacrificed his will to his machinery already; there was nothing for the city to take.

And some were driven into madness. Cort deteriorated rapidly after his explosion at the séance; I saw little enough of him, indeed I tried to avoid him, but could not but notice how he looked more haggard by the day, heard reports that he was receiving visitations from his phantom more often. He worked obsessively, but got nothing done; until then he had actually been making progress. Macintyre's internal buttressing was all but complete. But now most of his workforce abandoned him as his behaviour became so erratic they did not want to come near him. So he worked alone, furiously making drawings that no one would execute, ordering materials that lay in the courtyard untouched until he sent them back and began an argument with the supplier.

'Is Cort insane?' I asked Marangoni. I fully anticipated his reply, and was astonished by what he in fact said.

'Well, do you know,' the doctor replied in his heavy accent, putting the tips of his fingers together to look more professional, 'I do not think he is. Unbalanced, certainly. But I do not think he is insane. His mother's name was Annabelle,' he said, in total breach of the normal notions of discretion. 'She died when he was born, and he worships her memory. The idea that she was displeased with him shook him to the core. He told me this a couple of days ago.'

'You're still seeing him?'

'Oh, yes. It is vital considering his state of mind. He was in the hospital for the better part of a week, and I thought it a good idea to get him to come for a regular chat. He finds it peaceful just to sit in the sun looking at the lagoon, undisturbed. He goes away calm and contented. Normally. Sometimes we find him a bed here. We have a guest house, you know. A strange arrangement, but the monks were very hospitable, and for some reason we keep up the tradition.'

'Have you worked out what happened at the séance? Do you think the Marchesa did it deliberately?'

'I'm sure she is entirely genuine in her beliefs,' he said with an indulgent smile at the foolishness of women unaccustomed to the rigours of the scientific method. 'The trouble is she is in many ways a very stupid woman. She will hear something then forget it entirely. She has a very poor memory. But it remains in her mind, and when it pops up again, she believes it is a spirit which has told her. I'm sure she was told that Cort's mother's name was Annabelle, but forgot it. Then it came back to her.'

'You seem to think he will recover.'

Marangoni shrugged. 'That depends on what you mean by recovery, of course. If all matters which might disturb him were removed, I dare say he would soldier on. The trouble is that this is unlikely. He should return to England immediately. If he stays here, then perhaps not.'

'But is he safe? His behaviour . . .'

'. . . Is the behaviour of a madman. I grant you that. But does that mean he is insane? I have already told you how many people – women especially – are mad while showing none of the symptoms of madness. So we must equally consider the possibility that someone who behaves as though he were mad might, in fact, not be.' He smiled.

I stared, quite uncomprehendingly, at him.

'Perhaps his behaviour is a perfectly reasonable response to his current situation,' Marangoni suggested quietly. I knew exactly what he meant.

'He stabbed his wife with a knife. Are you telling me she deserved it?'

'Oh no. I very much doubt anyone deserves to be stabbed. He may – at that particular moment – have considered she deserved it; that by striking at her, he was warding off the torments he was experiencing. Of course, this was heightened by the drugs.'

'What?'

'Oh, you people!' He said with exasperation. 'You really notice nothing, do you? Did you not see the glassy eyes, the sweating, the slurred expressions, the way his movements became more uncontrolled and exaggerated?'

'I thought he'd been drinking.'

'He drank nothing but water. Opium, my dear Stone. Classic symptoms.'

'Cort is an opium addict?'

'Dear me, no. But he had undoubtedly consumed some of the drug shortly before he arrived. It is easy enough to come by. You can buy it in most pharmacies.'

'He told you this?'

'No. He denied it absolutely. Nevertheless, he was certainly under its influence.'

'So, he's lying. Perhaps he is ashamed.'

'Perhaps he was unaware of it,' Marangoni said absently. 'Not that it matters. He will get no more of it while he is in my care.'