‘We are trying to help you,’ said Holmes. ‘Act quickly!’

‘I’ll go first,’ said Leo Vitale, opening the window and stepping through it onto something. ‘There is a trick to it. You must be careful.’

We followed the young man and clambered out of the window and onto a rickety iron construction. I wondered what had occasioned Vitale to exit this way before.

The spidery, long-limbed student and the always spry Holmes had no trouble navigating the rickety ladders heading to the ground, despite a railing that had come loose and two missing steps. I struggled but managed to follow.

We found ourselves in an alley behind St Cedd’s court, safe but only for the moment. The oppressive heat seemed to settle into the grime and rubbish around us and be reflected back from it.

‘I am going to the laboratory,’ said Vitale.

‘They will soon look for you there,’ said Holmes. ‘Is there no other place we can take refuge?’

Vitale shook his head. ‘If I am to be arrested, I must secure my papers, first. Do what you like, but that is where I am going.’

After a quick and frantic run through Cambridge’s back alleys, we arrived, drenched in perspiration, at a back door of the Cavendish Laboratory. Once inside, we raced down a cool hallway and into the large room where we had first encountered Vitale. The windows of this laboratory were now strangely blacked out with thick fabric. Vitale threw a switch and electric light flooded the room.

Holmes looked around. ‘No way out. I don’t like it. Is there another egress, Vitale?’

The boy did not answer but focused on his business. He opened a desk drawer, reached deeply in it to the back and removed a stack of papers. He hesitated, looking around the room, evidently for a place to hide them.

‘There.’ Holmes pointed to a ventilator grate high on a wall near the sink.

Vitale nodded, clambered onto the stone counter nearby and stuffed them inside the vent, where they could not be seen.

As he did so, I noticed that in addition to the covered windows, something else had changed in the laboratory since our last visit. Rows of glass tubes shaped like long, thin sausages now crisscrossed along the walls, all dangling by wires and making long patterns of what looked like random, gigantic ant trails.

‘Aha’ said Holmes regarding this same display. ‘This is interesting.’

‘Pah!’ said Vitale. ‘This is Cosimo’s work. My senior lab partner has lost the story!’

What, I wondered, did that mean?

Satisfied that he had secured his research notes, Vitale leaped nimbly down from the counter and approached us.

‘And now, Mr Holmes,’ he said. ‘I … I will wait no longer. Tell me all, sir. What happened to Dillie? Please. How did she die?’ He swallowed, his face hardened, and he placed a hand on the stone counter, bracing himself for the news.