Q: What species is "put me on the serious" (1744)?

[Joel S. Berson]

I came across the following, and am wondering how to classify it.

"This Contrast in these two, and the eagerness with which they 
Espous'd their Favorite Topicks one of Praising, and the other of 
Blaming, put me on the Serious, to Consider the Motives from which 
they both Acted".

"Journal of William Black, 1744".
In Pennsylvania Mag. Hist. & Biogr. (1877) I. [no. 4] [410]

I' tempted to call it "serious, adv., sense 2, antedates OED3 
1853--", but other opinions are welcome.

There are 3 ghits in Google Web for "put me on the serious", but in 
each the form is "put me on the serious <noun>" (egg street, stuff, 
and backburner).  My at-home resources are limited.

Joel  

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