Gases

Historically, the study of gases was vital in eastablishing atomic theory, the idea that matter is made of tiny particles instead of a continuous mass. Practically, the study of gases is vital in understanding that rather useful cocoon we call the atmosphere. And empirically, theoretically, and mathematically, the study of gases reveals some straightforward relationships (such as that between a gas's volume and temperature) that are vital to the broader study of chemistry itself (such as, oh, say //absolute zero// and //kinetic molecular theory//).