Anyway...
The glassware remained innocent of the brown-black sheen that, according to The Compound of Alchemic, would indicate arsenic in the sample. She'd tried every type of food and drink she could find in the palace pantries, and pressed into service every bottle and jar she could discover in the Watch House.
She tried one more time with what said on the packet it was Sample #2. Looked like a smear of cheese. Cheese? The various fumes thronging around her head were making her slow. She must have taken some cheese samples. She was pretty sure Sample #17 had been some Lancre Blue Vein, which had reacted vigorously with the acid, blown a small hole in the ceiling and covered half the work-bench with a dark green substance that was setting like tar.
She tested this one anyway.
A few minutes later she was scrabbling furiously through her notebook. The first sample she'd taken from the pantry (one portion of duck pate) was down here as Sample #3. What about #1 and #2? No, #1 had been the white clay from Misbegot Bridge, so what had been #2?
She found it.
But that couldn't be right!
She looked up at the glass tube. Metallic arsenic grinned back at her.
She'd retained a bit of the sample. She could test again, but... perhaps it would be better to tell someone...
She hurried along to the main office, where a troll was on duty. 'Where's Commander Vimes?'
The troll grinned. 'In der Gleam... Little-bottom. '
'Thank you.'
The troll turned back to address a worried-looking monk in a brown cassock. 'And?' he said.
'Best if he tells it himself,' said the monk. 'I only work on the next bench.' He put a small jar of dust on the desk. It had a bow tie around it.
'I want to complain most emphatically,' said the dust, in a shrill little voice. 'I was working there only five minutes and then splash. It's going to take days to get back into shape!'
'Working where?' said the troll.
'Nonesuch Ecclesiastical Supplies,' said the worried monk, helpfully.
'Holy water section,' said the vampire.
'You've found arsenic?' said Vimes.
'Yes, sir. Lots. The sample's full of it. But ...
'Well?'
Cheery looked at her feet. 'I tried my process again with a test sample, sir, and I'm sure I'm doing it right...'
'Good. What was it in?'
'That's just it, sir. It wasn't in anything from the palace. Because I'd got a bit confused and tested the stuffl found under Father Tubelcek's fingernails, sir.'
'What?
There was grease under his nails, sir, and I thought maybe it could've come from whoever attacked him. Off an apron or something... I've still got some left if you want a second opinion, sir. I wouldn't blame you.'
'Why would the old man be handling poison?' said Carrot.
'I thought he might have scratched the murderer,' said Cheery. 'You know ... put up a fight...'
'With the Arsenic Monster?' said Angua.
'Oh, gods,' said Vimes. 'What time is it?'