(see the previous post, if you haven’t read it already)
There is the reddish glow of the warm sunlight. And, quite generally, the colors of things. But let us take one step after the other.
First, it was warm (it really was a nice summer day when I took that picture). What does it mean for something to be hot or cold, on a basic level? Again, suppose you meet your scientist at the Tuileries opening in the middle 1500s. What would have been the then-state-of-the-art ? Pretty lousy, it turns out!
Thermometers were developed only around 1600 (among others, by Galilei), although the principle that hot substances tend to expand had been known even to the Greek. And even if you have a working thermometer, you still wonder: what is the basic reason for something to appear hot or cold?
The first one to really get it right was Daniel Bernoulli, and he explained the idea in his book Hydrodynamica in 1738. His idea is simple and beautiful: Heat is motion. A gas is made up of billions and billions of molecules. These are not at rest, but constantly moving. Their energy is a direct measure of temperature. The faster they are, the higher the temperature. The same happens in solid substances or liquids, where constantly a kind of “jitter” motion is going on, with particles bouncing back and forth, never really at rest. When a cold body receives heat from a warmer one, its particles start moving around faster. All of this you cannot see directly, because the particles (atoms and molecules) are a thousand times smaller than what even the best light microscope would resolve, but you are witnessing the effects of this microscopic motion by feeling the temperature change.
From there on, the theory of thermodynamics and statistical mechanics continued to develop (at first rather slowly, it must be admitted). A lot of useful insights resulted. For example, in the beginning of the 19th century, a French engineer called Carnot realized that you cannot convert heat entirely into useful mechanical work. That means there are fundamental limits to the efficiency of power plants. Beginning in the middle of the 19th century, Maxwell and Boltzmann put the ideas of Bernoulli about the gas particles on a more quantitative level. All of our microscopic understanding of the properties of materials or the workings of living matter (cells) rests on the principles discovered back then.