Atkins begins not with the first law, but with the "Zeroth." Historically, this law was formulated after the first and second laws, but scientists realized it was so fundamental that it had to come logically before them.
The Statement: If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
The Implication: This allows us to define temperature. Without the Zeroth Law, thermometers would be meaningless. When you search for the .PDF of this book, you are essentially looking for a guide that explains why a thermometer works. Atkins uses elegant language to describe how temperature is the "potential" for heat flow—just as a voltage is the potential for electric current. Four Laws That Drive The Universe By Peter Atkins -.PDF-
In Atkins’ view, the Zeroth Law imposes a structure on the universe. It tells us that the universe is logically consistent. If object A feels cold to object C, and object B feels cold to object C, then A and B are the same temperature. This law drives the universe by allowing thermal contact to eventually lead to a uniform state—a state of "lukewarm death," which is a prelude to the Second Law.
Reading the .PDF is an intellectual exercise, but applying the Four Laws is a life philosophy. Atkins begins not with the first law, but with the "Zeroth
Atkins uses these laws to argue that the universe is not a machine that runs on time, but rather that time itself is a byproduct of these laws.
Atkins uses the metaphor of a bank account. You can move money (energy) between checking and savings (potential and kinetic energy), but the total sum remains constant. This law dismantled the dreams of alchemists and perpetual motion machines. Reading the
However, Atkins points out the "tragic flaw" in this law. While the First Law tells us that energy is conserved, it doesn't tell us how it will be used. It offers no preference. A cup of coffee cooling down conserves energy just as well as a cup of coffee heating up. To explain why one happens and the other doesn't, we need the next law.