When we asked Wayne State University Professor of Industrial/Organizational Psychology Dr. Marcus Dickson to shed some light on the when the sport once known as base ball became known as baseball, here's what we learned:
There were, of course, many bat and ball games prior to our modern version, and they went by many names in both England and the U.S. (and elsewhere). Virtually all references to a game with "base" and "ball" in England, and almost all known 18th and early 19th century references in the U.S., refer to the game as "base-ball". For example, the famous 1791 ban on game playing that threatened the windows of the Meeting House in Pittsfield, Mass., says "no person or inhabitant of said Town, shall be permitted to play at any game called Wicket, Cricket, Base-Ball, Bat-Ball, Football, Cat, Fives, or any other games played with Ball, within the Distance of eighty yards from said Meeting House."
By the late 1830s and into the 1840s, "base ball" had begun to replace "base-ball." Interestingly enough, the Knickerbocker rules of 1845 and the amendments from 1854 do not include either version of the word -- they simply refer to "the playing rules for 1854", etc. But the rules amended for 1857 do explicitly refer to "base ball." Nothing other than simple changes in type-setting convention seems to have driven the change, as far as I can tell (as an analog, we no longer use "&c", but instead use "etc.", for no particular reason), and in many cases the decision on what form of the word to use seems not to have been made by the authors, but by the typesetters and their chosen convention.
Just for fun, the convention changed again (after "base-ball" had become "base ball"), and by the mid-late 19th century (late 1870s, early 1880s), the word in the U.S. had again become "base-ball", and according to Dickson's (no relation) Baseball Dictionary (a major source for much of this essay), in 1896 the U.S. Government Printing Office specified that the word should be spelled with a hyphen. Again, no reason for the change is given or evident.
However, by the time the U.S. Government took action (no surprise here), the field was shifting again, and The New York Times had explicitly changed its style guide away from "base-ball" to "baseball" in 1884. As always, no reason other than common usage is given.
Of course, these practices overlapped in time, and you can find the word "baseball" (combined, no hyphen) in print as early as 1858 (though not at all common, and actually as early as the 1830s - though that is believed to be a misprint). You can find "base ball" in some circles into the 1900s (again, not at all common). So there are periods of overlap, and some periods of pretty common usage, and 1867 is square in the middle of a period in which "base ball" was the common spelling. However, I don't think that they ever got as worked up about it as we do today, in our efforts to be accurate. Yes, it was clearly "base ball" in 1867, but any person reading "base-ball" or "baseball" at that time would have known what was intended. In our efforts to highlight the game's history, we emphasize the two-word spelling.
That's Marcus Dickson, playing catcher for the Lah-De-Dahs.