Which preposition to use with mercurial

as Occurrences 1%

This youth of whom we are now to speak was such a mercurial as the like hath seldom been known, and could make his own part if at any time he chanced to be out.

at Occurrences 1%

BAROMETERSThe Goldsmith form of direct-reading aneroid is the most accurate portable instrument and, of course, should be compared with a standard mercurial at the last weather-bureau station.

Which preposition to use with  mercurial