I was fascinated by this little bit from the author, since she refers to venting her spleen! I did a six novel search and came up with three other references to spleen. (I know, I'm interested in the oddest things!) It's also an interesting passage since she acknowledges the reality of hating "people whose parties or principles do not suit with mine..." Seems a rather advanced realization from someone so young.
====Henry VI====
if you do not, you had better read some other History, for I shall not be very diffuse in this, meaning by it only to vent my Spleen against, & shew my Hatred to all those people whose parties or principles do not suit with mine, & not to give information
====Emma: ====
Emma shall be an angel, and I will keep my spleen to myself till Christmas brings John and Isabella.
====Mansfield Park: ====
It raises my spleen more than anything, to have the pretence of being asked, of being given a choice, and at the same time addressed in such a way as to oblige one to do the very thing, whatever it be!
====Pride and Prejudice: ====
Adieu to disappointment and spleen.