This video (https://www.youtube.com/watch?v=ddx_nsYos1A) from the Royal Society quotes a letter from the 1780's which seems to use 'is a thing' in the modern sense. The reference starts at 04:00.
Also, the site you link to above is at the University of Pennsylvania, not Penn State.
Also, the site you link to above is at the University of Pennsylvania, not Penn State.