From Othello (3.3.329-332):
Dangerous conceits are, in their natures, poisons.
Which at the first are scarce found to distaste,
But with a little act upon the blood.
Burn like the mines of Sulphur
And a similar notion, less direly construed – from Tristram Shandy v.2 ch.19:
I mention this, not only as matter of hypothesis or conjecture upon the progress and establishment of my father’s many odd opinions,—but as a warning to the learned reader against the indiscreet reception of such guests, who, after a free and undisturbed entrance, for some years, into our brains,—at length claim a kind of settlement there,——working sometimes like yeast;—but more generally after the manner of the gentle passion, beginning in jest,—but ending in downright earnest.