I found a copy of the original letter on an Apriarian sect here: http://memory.loc.gov/master/mss/mtj/mtj1/051/0500/0555.jpg.
This is addressed to Thomas B. Parker. My transcription is as follows. I hope I got it right:
Monticello May 15, 1819
I thank you, Sir, for the pamphlet you have been so kind as to send me on the reveries, not say insanities of [...]



