‘The lab thought we might want to know that. Killing tree stumps, for a start. You can get it in most garden centres, or hardware shops. It’s an ingredient of some fertilizers. Also toothpastes that are formulated for sensitive teeth. And gunpowder.’

‘Versatile stuff, then.’

‘Wait – you haven’t heard the best one. Potassium nitrate was considered for many years to be an anaphrodisiac.’

‘A what?’

‘They thought it suppressed sexual desire. It was added to food in all-male institutions. Did you ever see One Flew Over the Cuckoo’s Nest?’

‘The film with Jack Nicholson?’

‘Right. Well, in the film, Nicholson’s character mentions that’s he’s afraid of being “slipped” potassium nitrate in the mental institution where he’s committed. It was a common practice at the time, a way of controlling the behaviour of patients.’

‘Cool. But I’m not sure it helps us.’

‘Is potassium nitrate a natural product, or artificially manufactured?’ asked Cooper.

‘It’s a naturally occurring mineral,’ explained Hitchens. ‘Traditionally, the major sources were the deposits crystallizing on cave walls, or the drainings from dung heaps. Ammonia from the decomposition of urea – you know.’

‘We get the picture,’ said Fry.

‘But it can also be manufactured. The old method was to mix manure, wood ash, earth and organic materials such as straw.’

Cooper nodded. ‘A compost heap, in fact.’

‘Exactly. But they were known as nitre beds, which sounds nicer, I suppose. A heap was kept moist with urine in the, er … traditional manner, and turned to accelerate decomposition. After a year, it was leached with water, and the resulting liquid was rich with nitrates, which could then be converted to potassium nitrate, crystallized and used in gunpowder.’

‘Potassium nitrate is an explosive, then?’ said Fry.`

‘Not on its own. It does have another useful property, though, particularly for fireworks manufacturers. A mixture of potassium nitrate and sugar produces a smoke cloud six hundred times its own volume. Just great for smoke bombs.’

‘How does it kill tree stumps?’

‘It doesn’t really kill them. You have to kill the stump first – then the potassium nitrate makes it decompose faster.’

‘Wait a minute,’ said Cooper. ‘Those compost heaps – did you say they were called “nitre beds”?’

‘That’s right. One of the common non-scientific names for potassium nitrate is –’

‘– saltpetre?’

‘Correct.’

Cooper found that he wasn’t in the least surprised. It seemed to fit so naturally with what he’d learned already of the owners of Pity Wood Farm, and the other residents of Rakedale.

‘Potassium nitrate is used in Edgar Allan Poe’s story “The Cask of Amontillado” as the lining of the crypt where Montresor buries Fortunato alive. It’s why he has so much trouble with his breathing in his last moments. “For the love of God, Montresor!”’