Norman Vialle, a 53-year-old car dealer in Kansas, invested in his share of winners and losers during the Internet bubble of the 1990s. Now he is clinging to a stash of Bitcoin, even though the fledgling virtual currency has lost about 70% of its value in the past week.

"It's volatile because it's new, but it's still a lot higher than it was a month ago," Mr. Vialle says.

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