The Molecular Tactics of A Crystal by Lord Kelvin

My subject this evening is not the physical properties of crystals, not even their dynamics; it is merely the geometry of the structure—the arrangement of the molecules in the constitution of a crystal. Every crystal is a homogeneous assemblage of small bodies or molecules. The converse proposition is scarcely true, unless in a very extended sense of the term crystal. I can best explain a homogeneous assemblage of molecules by asking you to think of a homogeneous assemblage of people. -an excerpt