Around 1700, Isaac Newton (1642 - 1727) applied his mind to the problem of heat. He elaborated a first qualitative temperature scale, comprising about twenty reference points ranging from "cold air in winter" to "glowing coals in the kitchen fire". This approach was rather crude and problematical, so Newton quickly became dissatisfied with it. He knew that most substances expand when heated, so he took a container of linseed oil and measured its change of volume against his reference points. He found that a liter of linseed oil at the temperature of melting snow grew to 1.0725 L at the temperature of boiling water.
After a while, he defined the "zeroth degree of heat" as melting snow and "33 degrees of heat" as boiling water. He called his instrument a "thermometer".
- Photo of an antique thermometer backing board c. 1758—marked in four scales; the first is Newton's.