Using his thermometer, Fahrenheit was able to determine the boiling points of liquids and found that they vary with atmospheric pressure.

His first thermometers contained a column of alcohol which expanded and contracted directly, as originally devised in 1701 by the Danish astronomer Ole Römer (1644-1710).

Fahrenheit substituted mercury for alcohol because its rate of expansion, although less than that of alcohol, is more constant and could be used over a much wider temperature range.

To reflect the greater sensitivity of his thermometer, Fahrenheit expanded Römer's scale so that blood heat was 90° and an ice-salt mixture was 0°; on this scale the freezing point was 30°.

Fahrenheit later adjusted the scale to ignore body temperature as a fixed point so that the boiling point of water came to 212° and the freezing point was 32°. This is the Fahrenheit scale that is still in use today.

For more details, see Centigrade as well as this Thermometer and Temperature Scale page.