"The mind, or genius has been compared to a spark of fire, which is smothered by a heap of fewel, and prevented from blazing into a flame: This simile which is made use of, by the younger Pliny, may be easily mistaken for argument or proof."
— Reynolds, Joshua (1723-1792)
There is no danger of the mind's being over-burthened with knowledge, or the genius extinguished by any addition of images; on the contrary, these acquisitions may be as well, perhaps better, be compared, if comparisons signified any thing in reasoning, to the supply of living embers, which will contribute to strengthen the spark, that without the association of more, would have died away.
(p. 14, p. 214-215 in 1778 ed.)
Text from A Discourse Delivered to the Students of the Royal Academy, on the Distribution of the Prizes, Dec. the 10th, 1774. By the President. (London: Printed for Thomas Davies, 1775). <Link to ECCO>
See also Sir Joshua Reynolds, Seven Discourses Delivered in the Royal Academy by the President (London: T. Cadell, 1778). <Link to Google Books>
Also reading at PGDP.