According to any accepted, logical understanding, one would think that the solid is the material state that is most easily investigated. As opposed to gas, it is visible; as opposed to fluid it doesn't evade your grip. Stable, compact, consolidated, it is allegedly much easier to handle than any other substance. The collection of solids began with the first stones of the cave men. In the course of the centuries, the list became constantly longer. And yet, despite all that, it is astonishing to realize that only within the past 50 years man has succeeded to determine, beyond any doubt, the structure of the solid.
Until this breakthrough, the understanding that prevailed was that it is the composition of the solid that determines its features: that it is its composition that makes diamonds hard, leather tough, iron magnetic, and copper conductive. Change the composition, and you'll be able to change the substance - this was the motto of the classic chemists. Add the appropriate substances, and you'll be able to create a new, completely different substance out of the source substance. Nowadays we know that many of the solid's features are actually determined by its structure, by the way its fundamental constituents - its atoms - are arranged, and by the way they are interconnected. We also know that the primary difference between the solid and the fluid or the gas - the feature that makes it look rather hard - is the arrangement and the relative proximity of its atom groups (or molecules).
The shift of the focus of interest from the solid's composition to its form has brought in its wake a giant leap in the course of the operation, during which man conquered his physical environment. From being a mere finder and collector of substances, he became the creator of new substances, the resourceful architect of the substance. He entered his laboratories and generated diamonds from peanut butter, silk from coal; he even created a rather acceptable wallet out of pig's ears. Many of his laboratories were themselves transformed. Miles away from the usual layman's image of the empire filled with test tubes and stones, scattered around in disorder, the modern scientist, dedicated mainly to the threedimensional structure of things, fills his realm with threedimensional models of iron wires, wood and plastics - winding and mobile cylinders and hemispheres, installations with rounded corners, interconnected with rods, greatly resembling the toys that can be found in any well equipped nursery. As the researchers investigate these models, trying to introduce all kinds of changes into them, they try to understand intuitively, how certain features of a substance are actually derived from the structure itself.
The long path of understanding the solid is lined with milestones of particular significance. The first milestone goes back to the beginning of the nineteenth century, when it became clear that many solids - and not only the transparent among them - are composed of crystals (some execptions are organic tar, vulcanic glass, and opals). We find the second milestone at the end of that century, with the appearance of the modern atom theory, which proved that the crystal itself was composed of specific arrangements of atoms. The third milestone, finally, was the late discovery in 1912, which proved that it is possible to examine the inside of a solid by means of the miraculous device called x-ray-machine (the x-ray had been discovered 17 years earlier). Being applied for this purpose, the x-ray provided the final confirmation of the image of the solid as an orderly mosaic of tiny crystals.
