.... Earlier today, the People's Bank of China warned that the controversial electronic currency carried risk. It also instructed financial institutions not to trade it.

"Is this a bubble in Bitcoin?" asked former Federal Reserve Chairman Alan Greenspan Wednesday on Bloomberg. "Yeah it's a bubble."

"This is worse than the tulip mania," said Nout Wellink, former president of the Dutch Central Bank. "At least then you got a tulip [at the end], now you get nothing."...