‘What exactly is this test?’ she asked. ‘I’ve heard of it, but I’m not quite sure how it works.’

‘It’s been around for ages,’ answered the biologist, always happy to instruct. She pointed to a rack of labelled vials, taken from the refrigerator. ‘These are made from the blood of rabbits immunized with sera from different animals, including humans. A small quantity is injected, which doesn’t hurt the rabbit, but stimulates the production of antibodies specific for the proteins of that particular beast.’

Sian nodded, she knew about antigen-antibody reactions from her time in the hospital laboratory.

‘How do you get a result, then?’

‘Basically, an extract is made from the test sample and layered on top of each specific serum in a tube. If an antigen for a particular species is present, then a white line appears at the junction between the two fluids, due to protein being precipitated.’

She added small amounts of the fluids into a series of tubes as she talked. ‘In this case, I’ll have to get rid of all this fat first and get a clear solution. Let it incubate for a few hours and see what happens. Naturally, we have to set up controls and blanks to make sure the result is genuine.’

It was the afternoon before all this was done and Priscilla was able to confirm that the substance was definitely human in origin by the time they assembled for tea in the ‘staff room’, between the kitchen and the staircase that went up from the centre of the hall.

‘I’d better confirm to the cops in Aberystwyth that they’ve probably got an unexplained body on their patch,’ said Richard. ‘They’ll have to inform the coroner first of all.’