Colorado Measure 19 (1912)
From Ballotpedia
Colorado Measure 19 (1912) was a measure creating an 8 hour law for work in underground mines, smelters, mills, and coke ovens. It passed.
The measure was one of twenty-two on the Colorado ballot in 1912, the first year that initiative rights were enabled in the state. Fourteen of the 22 measures failed, and eight passed.

