The other question was: which of the official residents was hosting this interloper? I knew that I was the last person to move in and, I was told by the landlord - apparently, we were "just one big happy family" - that everyone else had been resident for a year or more. I assumed that it must be someone in the house, since there was no direct access into the attic from the outside. It was a semi-detached building, but the dividing wall between the two properties was built right the way up to the roof, and indeed supported the roof tresses themselves.

Perhaps the most obvious candidate was the young man I knew only as Victor. He always seemed as poor as a church mouse, even by the demanding standards of postgraduate students. I understood from the occasional corridor conversation that his parents had fled from Iran after the fall of the Shah, having been forced to leave most of their wealth and possessions behind. Maybe he might have been supplementing his income by charging an acquaintance for some cheap accommodation.

The other possibility that occurred to me was the two aloof girls whose names I did not know. They shared a large room on the first floor, in what would once have been the Withdrawing Room.

I knew this would once have been a grand house, now fallen into - well, not disrepair, but more a state of minimal maintenance done on the cheap by a landlord more interested in securing a regular rental income than maintaining the integrity of the building.

The girls had irritated the other residents on a regular basis by repeatedly using the last of the toilet paper in the shared facilities, and then refusing to buy any more. In self-defence, we had fallen into the habit of keeping our own bog-rolls in our own rooms. The girls' cheapness led me to suspect that they were on the lookout for ways to save money.

The shared kitchens were another bone of contention in the household. There were two, on the ground and the top floors. These were shared facilities, with separate cupboards, each now fitted with a padlock, for each of the residents. No food or utensils was shared with any of the other occupants, and as little as possible was kept in the shared fridge - the little that was stored therein was always marked up with the owner's name in large letters.

So, there were always people moving about in the house, at almost any time of the day or night. Nevertheless, I did not recollect hearing any movement from the attic - no scrapes or taps, no sound at all, even though the makeshift bedroom was immediately above my room.

So it was with some surprise that, after my exploration of the loft, I began to hear noises above me, not the creak of the floorboards moving but soft susurrations which could perhaps be that particular deep breathing some people exhibit when sleeping - not really snoring - or could conceivably be someone softly crying.

I was at a loss for a while, wondering what to do. I tried looking up through the loft entrance a time or two, again standing precariously on the back of the chair. Invariably, there was nothing new to be seen, just the same neatly-arranged belongings. I was always slightly surprised that the light still worked, each time I switched it on, although I conscientiously switched off it again after each inspection.

After some further thought, I traded some of my junk for a video camera and a closed-circuit television monitor. I was by now familiar with several backstreet shops and market stalls where second-hand electronic equipment could be purchased for a few pounds, or even traded. I also had a variety of contacts at the University Amateur Radio Society.

It did not take long for me to acquire and test the necessary CCTV components. I also acquired an ancient and rather rickety wooden stepladder from one of those second-hand shops that seem to sprout like mushrooms in areas where impoverished students habitually reside.

Having assembled the kit, I waited with barely restrained patience for an afternoon when I believed I was alone in the house.

Firstly, I ran a couple of extra cables up into the attic, carefully enlarging the hole in the ceiling I had made previously. Then I carefully installed the camera in a spot which I hoped would give a clear view while still being reasonably unobtrusive.

I was up and down the stepladder several times: first to connect the cables to the monitor and the mains electricity supply and then, after I had got it all switched on, up again to fine-tune the position of the camera. Finally I was satisfied. I made my way back down the ladder, closing the trap door and carefully leaving the light on to provide enough illumination for the camera to work properly.

That evening, I was working intermittently on a long-overdue report, periodically looking up from my hand-written notes to stare at the monitor now sitting on the corner of my desk.

Perhaps the sixteenth or seventeenth time, I caught some kind of movement on the screen. It appeared to be smoke or perhaps dust, being blown about, and I would not have bothered with it if I had not noted that the weather outside was particularly still. As I watched with growing fascination, a figure formed from the dust in the air, lit by the light bulb I had omitted to turn off.

At first I thought that it must be some kind of interference - perhaps the monitor was picking up a television transmission, however unlikely that must have been - but I was soon convinced that the rather jerky black-and-white images on the tiny monitor screen really were showing what was happening a few feet above my head.

The figure was that of a young woman, crying with her face in her hands. She had the straight, rather lank-looking hair of someone with neither the time nor the money to spend on hairdressing, and she was wearing blue jeans and a shapeless sweatshirt that was the twin of one I had seen in the attic suitcase.

At first, the camera saw her from behind, sitting cross-legged on the floor, right next to the folded sleeping bag. As I watched, she turned to face the camera, her face lightening slightly to a wan smile. Then there was a faint click and the screen went black.

My first reaction was to twiddle the knobs on the monitor, but it rapidly became apparent that the screen itself was still working just fine and I strongly suspected the camera was OK too. The reason the picture was dark was that the electric light in the attic had been turned off.

It took me but a few moments to wrestle the heavy wooden stepladder from behind the wardrobe where I had hidden it, lug it across the room and set it up on the landing just outside my door. I gingerly pushed open the wooden trap door with my head, peering cautiously into the darkness.

There was nothing to see, just the dark space faintly outlined by the light from below. I could not hear any sound or detect any kind of movement. I cautiously reached for the light switch and flicked it, dim light once again flooding the attic space.

I could see nothing out of the ordinary. Everything was just as I had left it, the sleeping bag neatly folded, the battered suitcase of clothes closed. As before, everywhere was astonishingly dust-free, except for one spot, just where I had seen the apparition, where there was a neat conical pile of dust.

To this day, I am not sure why I did what I did next. I climbed back down the ladder and returned to my room, where I tore a page from my notepad. Then I detoured to the kitchen and, after a short search, found an empty jar in one of the cupboards.

So equipped, I returned to the attic and used my hands to sweep the dust onto the sheet of paper, using it as a makeshift funnel to tip the dust into the jar. Having tidied the attic space, I returned to my room, again taking care to leave the attic light switch on. I held up the jar of dust I had just collected to what light there was; I could see nothing unusual and I simply plonked it down on the nearest available flat surface, which happened to be the top of the CCTV monitor.

I kept watching the camera for several days, while I was at home working, but there was nothing further to observe. The attic light burned without interruption, and there was no sign of movement on the monitor screen. Intrigued nevertheless, I made some more enquiries, taking the time to loiter in the corridors and chat with the other residents as they went about their lives. Eventually, it was Victor who told me that there had been a female resident, whose name he recalled as Angela, in the room I had moved into just a few weeks ago, but she had not been the most recent prior inhabitant.

The immediately previous occupant had been a morose and taciturn young man named Graeme, studiously quiet and one who very much kept himself to himself. Victor thought him to be a student at the University, but neither he nor anyone else I spoke to could quite remember which Department or School he attended.

Victor suggested that Graeme had been the boyfriend of the girl Angela. She had disappeared from the household quite suddenly, and all her stuff went too. He had simply assumed she had just moved out and Graeme had moved in, with some kind of agreement between the two of them.

It was late in the evening after Victor had related his intelligence when I heard noises in the night again. I was awoken suddenly from a deep sleep by what sounded, as I lay there in the dark, like a soft voice lamenting, but I could not make out the words.

Deeply intrigued, I turned on the bedside light, then went over to the desk, switched on the CCTV monitor and waited for it to warm up. After a minute or so, the image had stabilised and I inspected the screen closely, but I could see no movement in the dimly-illuminated attic.

As I leaned forward over the table, I noticed with increasing apprehension that the noises were not coming from the ceiling as before - they were emanating from the jar of dust I had stood on top of the monitor. I must have been incorrigibly curious. Without hesitation, I opened the glass jar and poured out the contents onto another sheet of lined paper I had torn from my notepad and placed on the floor.

I suppose I was not entirely surprised, nor indeed really frightened, when the dust began to move, in spite of the entirely still air in my room, swirling and forming into the same feminine shape I had seen in the camera.

She spoke in soft susurrations, the same sound I had heard above my room over the last few nights, only just audible even in the midnight stillness.

"Thank you for rescuing me."

"You're Angela, aren't you?" I asked, also speaking very softly, perhaps afraid of disrupting the delicate movements of the dust.

She nodded, watching me with wide eyes. She gave an astonishing impression of a real and very physical presence, even though I could see the light from my desk lamp shining through the dust particles which delineated her form.

"And you're dead, right?"

Again, she nodded, looking calmer.

"Why are you here?" I demanded shakily, adding, "And what do you mean, rescuing you?"

The cross-legged figure of dust told me a sad and perhaps rather predictable tale. She had had endless arguments with her long-term boyfriend Graeme, mainly because she refused to share a bed or even a room with him. Apparently, he was always abjectly apologetic in the mornings, and she really did love him, she said. In any case, she had taken to sleeping in the attic, in the sleeping-bag - mainly to stay away from his nightly wandering hands.

The tensions in the relationship could not help but blow up sooner or later, and one evening there was a huge argument. Graeme struck her, again and again, and she fell knocking her head on the brass bed-head, killing her instantly.

It turned out that Graeme was a postgraduate student in the Chemistry Department, and had managed to dispose of her body by dissolving it in a bath of strong acid in the basement of the Chemistry Department. I vaguely remembered hearing of tales of a giant theft of chemicals from the University stores.

To my growing horror, I understood that Graeme had then morbidly filtered and dried the residue from the acid bath, brought it back to this house - to this very room! - and had then taken it to the attic and poured it over her sleeping bag.

"I should go to the police," I exclaimed, louder than I had intended and sitting back suddenly.

Angela winced, the dust motes swirling either in agitation or because of the air currents my sudden movements had caused.

"The police won't believe you," she said sadly, "What evidence do you have? I'm just a kid, a teenager who's gone missing, like a thousand others. My parents disowned me years ago, and no-one else really going to look that closely."

"So what can I do?" I asked, suddenly deflated.

"You have to help me," she said, smiling radiantly.

"Bring him to justice, you mean?"

"It must be true and final justice," she answered enigmatically, "Otherwise he might escape punishment."

"But how?" I wanted to know.

Angela explained that she had been a student of biology. She knew that recent developments in the understanding of the structure of the human genome meant that it might still, in principle, be possible to reconstruct her genetic identity from this dust, to at least prove that she had been killed.

I didn't know, then or now, whether she was predicting the development of computerised DNA matching, capable of matching the fragments available in the dust against samples from a suspect.

"I just needed to communicate," she concluded, "To tell my story to someone who cares, someone who will, I am quite sure, take action on my behalf."

And with that, she quietly disappeared, never to be seen again, leaving just a thin layer of grey powder gently settling on the thread-worn carpet of my room.

*

The Professor was silent for a long moment, watching me carefully for, I supposed, my reaction to the incredible story he had just related.

"I suppose all this goes a long way to explain why I became a Professor of Computer Forensics and Criminology," he said slowly, "And, looking back, I now recognise that a defining success in my early career was the jailing of a violent husband who beat his wife to death. I was never able to prove it - it turned out that there was just not enough genetic evidence remaining - but I suspect that man was also Angela's killer."

The Professor leaned forward and again tapped his pencil on the glass container.

"And it also explains why I have a jar of dust on my desk."

Part 1 Afterword