Thank you all for your help with these queries. "impossible" fits perfectly, and the suggestions of "accuity / acuity" also fit, in the sense of clarity and sharpness. Sorry for the slow response - I missed the notifications that I had a reply.
I have one other request in the same hand please, and it's something to do with 'malice'. The letter dates to about 1831 and is English:
"...from being exposed. I have heard your Lordship tell a certain Peer in the House of Lords that his language was malice [_______]. I can safely say..."