What is one piece of evidence for Heat NOT being a fluid?

Answer 1

steam

Answer 2

You know, that's a really good question, because 19th century physicists had a hard time with this one. Ludwig Boltzmann was able to explain entropy and the 2nd law of Thermodynamics by positing a kinetic theory of heat, and still a lot of physicists weren't convinced. The "caloric" theory of heat, as it was called back then when physicists believed it to be a fluid, took a while to finally expire and yield to modern particle and quantum physics.

Part of the trouble is that statistical kinetic energy does flow very much like a fluid. Some early physicists had argued that heat "couldn't necessarily be contained in some fluid", because friction does produce heat. Rubbing two cold pieces of metal can produce heat, and so where did this "fluid heat" come from? Nonplussed, physicists were ready to believe in conversion from work into "heat energy", which was then "contained in a fluid" To kill off this "fluid heat caloric theory", we need to appeal to conservation of energy, and in a jar of liquid, for example, we can already calculate the total energy it has, knowing its atomic mass and average molecular kinetic energy (and rotational energy), which is directly measurable. That would account for all of its supposed heat energy, and leave no room for an independent "fluid body containing heat energy".

Answer 3

can't mix with vodka