JSTOR call it their “Early Journal Content”, and it includes journals and papers published before 1923 in the US and before 1870 elsewhere. It's about 6% of their collection and includes around 500k articles on economics and politics, arts and humanities, and in mathematics and other sciences.
While some might think it great that they are giving away content, I look at it another way. Up until this week JSTOR charged an admission fee so students and researchers could view content that JSTOR did not have any right to.
If that doesn't upset you, consider this: there is a percentage of the journal articles in JSTOR which were originally funded by federal and state grants. Basically JSTOR are charging for access to content that my tax dollars already paid for.
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