Both Uses

capricious
in
Frankenstein - 1831 version
(Edited)

We felt that they were not the tyrants to rule our lot according to their caprice,
p. 39.4 *

caprice = impulsive moods
By one of those caprices of the mind which we are perhaps most subject to in early youth, I at once gave up my former occupations, set down natural history and all its progeny as a deformed and abortive creation, and entertained the greatest disdain for a would-be science which could never even step within the threshold of real knowledge.
p. 43.3

caprices = instances of impulsiveness

Definitions:

(1) (capricious)
impulsive or unpredictable or tending to make sudden changes -- especially impulsive behavior
(2) (meaning too rare to warrant focus)