The Heat Atmosphere Around Dalton's Atoms
Almost every Chemistry textbook begins the discussion on atoms by introducing Dalton's atomic theory. They focus on some of Dalton's ideas - atoms of each element having a signature atomic weight, atoms of different elements combining in simple whole number ratios to form a molecule of a compound (or a "compound atom" as Dalton called it), etc.
One aspect of Dalton's theory that is not usually discussed is how he imagined the interaction of heat with his atoms. These were the days when heat was still considered to be a material substance. Lavoisier had even listed heat in his table of elements. Scientists at the time thought of heat as a "subtle fluid", since it did not display the properties of other fluids such as mass.
Dalton used his atoms to explain common observations about the different phases of a substance. He speculated that the atoms of any substance naturally attracted each other. So what caused substances to exist in the more fluid states?
"Besides the force of attraction, which, in one character or another, belongs universally to ponderable bodies, we find another force that is likewise universal...namely a force of repulsion. This is now generally, and I think properly, ascribed to the agency of heat. An atmosphere of this subtle fluid constantly surrounds the atoms of all bodies, and prevents them from being drawn into actual contact."
Dalton imagined heat as an atmosphere or a blanket of subtle fluid around the atoms. The heat atmospheres of different atoms repelled each other. He felt that this hypothesis neatly explained why substances expanded on being heated. Heating meant making the atmosphere of subtle fluid thicker, which increased the distance between the atoms.
He did notice a quantitative difficulty with his heat atmospheres. Scientists at the time knew of and accepted the idea of absolute zero temperature, at which all heat is removed from a substance. Given that a substance like water existed in its solid form far above absolute zero, it had to be believed that atoms in solids retained a significant proportion of the heat atmospheres that they had in the gaseous state. Dalton, was also unable to explain why some gases could be liquefied by simply compressing them with external pressure.
