He was trying to find a salt mixture that went through a phase change at 0 degrees: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/zero-fahrenheit.

He was trying to find a salt mixture that went through a phase change at 0 degrees: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/zero-fahrenheit.shtml http://antoine.frostburg.edu/chem/senese/101/solutions/faq/z... That wasn't so reliable, and neither was using 100 for human body temperature, so when he discovered that water boiled at 212 he began calibrating with that.