Dave posted about this yesterday. Ngrams is a beta tool from Google that lets you search a word or group of words in 15 million (12% of all published) books. Read this really detailed account of the project (HT Chris Blattman).
I figured I would plug in all the current African countries to see what comes up. 1880 was picked as a starting point since it was roughly when the upwards ticks in mentions of countries began. Sadly, I cannot do all of them on one graph. So, be aware that the %s are relative to those in each group. The first graph has an upwards of .001% while the next one is .005%. (Click on the images to see them a bit bigger)
For fun, does anyone want to draw some conclusions?