The rise and fall of mercurial

I used the word "mercurial" in conversation tonight. Then, a couple of hours later, I started a new chapter in the book I'm reading right now. And in one of those funny coincidences, there it was, just 138 words in: "mercurial."

That got me thinking: how common is "mercurial," anyway? I'm not pretentious enough to say it's a word I use often, but I am pretentious enough to say it's a word I like.

So off I went to the recently launched Google Ngram Viewer, which may be the greatest development in word geekery since the Oxford English Dictionary. Here's what I learned:

• In Google's vast archive of scanned books written in English, statistically noticeable use of "mercurial" first occurs in the late 1600s, mostly in metallurgy texts and proceedings of the Royal Society in England.
• The first big spikes appear from about 1720 to 1750. But the word really takes off in popularity around 1775—it's used in 0.0006% of the Google Books sample—evidently in works describing the usefulness of mercury-based treatments for venereal disease. (Mercury beats Venus: who knew?)
• After 1800, use drops sharply and never again rises to the heady heights of the 1700s. There was a bit of a resurgence in the 1850s and 1880s, probably because of increasing attention to the not-so-pleasant side effects of mercury on the human body.
• Since then, "mercurial" has been dropping steadily into obscurity. In this decade, its use in books has diminished to 1/12 the rate it enjoyed at its peak in the late 18th century.

By Google's tally, "mercurial" is used in a paltry 0.00005% of English-language books nowadays. That's one in 2 million. If its use in conversation runs to the same percentage (though I suspect that the actual rate is a hell of a lot lower), the odds are that only 16 other Canadians used it tonight.

I'm also pretentious enough to say I'd like to meet those people.