I have downloaded an Emerson concordance, and I have found a letter (apparently written in 1843) that is listed as part of Emerson's Uncollected Works, which may be the source you are thinking of.
http://www.emersoncentral.com/a_letter.htm
I hope this is what you are looking for, but please resubmit your question if it is not and you would like me to keep looking.