Google Ngrams is a curious little service that Google offers. It's like Google Trends or Google Insights, but instead of looking at the frequency that words appear (or rather are searched for) on the internet, it somehow calculates the frequency of words' appearing in books over the decades.
Who knows how they do it? Google is omniscient.
Here are a few NGrams. I'll add more every now and then. Remember they'll be more readily visible if you click on them.
This first one looks at the many names of the capital of China. Going all the way back to 1800, Pekin predominates in English-language literature. Around 1820, the terminal 'g' appeard, and for 70 years Peking and Pekin battled for supremacy. By 1890, though, it was all over, and the five-letter spelling became a mere typo as the six-letter name flew. The Pinyin spelling Beijing doesn't even appear until about 1975, but it takes off pretty quickly, surpassing Peking in only about ten years. By today, it predominates by far, as Peking fades away and Pekin sits barely above zero.
A nice one, this one. Over the years, our diets have turned from 'I love beef' to 'I really prefer chicken'. Do we write accurately about what we eat? Apparently so. Pork hasn't changed much down the years - for over a century it's been number three here - but beef and chicken have switched places dramatically, with the change happening around 1975. Not only is chicken number one today, but for some reason we're writing about it way more than we ever did about beef.
And lastly (this time out), the three opiates. I expected to see opium predominating in the distant past, as it does. But I didn't realise morphine was of such a vintage, going back to about 1840 and gradually inching toward opium's numbers. Heroin is more recent - though even it's on the scene all the way back to 1920. It surged recently, peaking in the late 1970s, but it surprises me how little that surge was. It very briefly peaked above opium and morphine before dropping down in the 1980s and 1990s, when all three were quite close to each other. After another nexus right before the turn of the century, morphine and heroin start to fade again, and if you can believe Google Ngram, by today it's opium back at number one. Huh.