If a substance is such that all the atoms which compose it are arranged on one and the same pattern, so that the straight rows run through from side to side, the substance is a single crystal; crystalline character meaning, simply, perfect arrangement. But most substances, and especially those we handle every day, such as the metals, must be described as masses of small separate crystals.
However we try to deform [a bar of multi-crystalline material], there are always some crystals which resist being deformed in that particular way. And the various crystals back each other up according to some principle which we do not fully understand. Thus the properties of the bar depend upon its crystalline character. It is only the X ray that can tell us the internal arrangement of the crystal.
The X rays are of short enough wavelength to be turned aside or scattered by the atoms, when longer light waves are not. A single atom can, however, do very little. Here is where the regularity of crystal arrangement comes in. The unit of pattern is repeated an enormous number of times even in a crystal just visible to the naked eye. Whatever one of these units does in the way of scattering, all the others do in regular order. The combined amount is perceptible, and so the crystalline character is detected.
Of course this is an indirect way of examining the structure. We do not perceive the individual atoms; we discover only their arrangements. But the knowledge so gained can be combined with other knowledge that we already possess and we have actually found ourselves able to decipher the patterns of Nature to an extent we did not dream of a few years ago.
The Bubble Chamber
By Donald A. Glaser (Nobel Prize in 1960)
Published February 1955
In their exploration of the submicroscopic world of atomic nuclei, physicists are like men groping in a dark cave with a flashlight that goes on for only an instant and each time lights only a tiny corner of the cave. Occasionally the flash catches some activity or event—either a familiar particle behaving in a familiar way or some strange new particle whose behavior is altogether baffling. From these scanty glimpses nuclear physicists are attempting to identify the particles and the forces at play in the dark, violent world of the nucleus of the atom. It would help if they had a better flashlight.
Let us look for a moment at the events they are trying to observe and at the observing devices that have been available up to now. Physicists are probing the nucleus by bombarding it with particles, preferably particles with enough energy to break up the nucleus into its constituent parts.
He has had two ways of seeing and measuring these happenings. The first is the Wilson cloud chamber. In a chamber supersaturated with a vapor, a flying charged particle leaves a visible trail of liquid droplets, which condense on the ions the particle has produced by hitting vapor and gas atoms in its path.... Sometimes the particle breaks down ("decays") into lesser particles which make divergent tracks. But these interesting events occur only rarely in a vapor-filled chamber, because collisions in the gas are infrequent.
The second device for recording nuclear events is the photographic emulsion. A particle charging into the dense emulsion has a high probability of colliding with nuclei; hence there is a good chance that the emulsion will show interesting events, including scattering, disintegrations and the formation of new particles. However, the emulsion also has its drawbacks. Its very density makes collisions so frequent and the particle path so crooked that the effect of a magnetic field cannot be measured. And from the moment it is manufactured an emulsion begins to collect random particle tracks from cosmic rays and terrestrial radioactivity
Could some compromise be found which would eliminate the defects and combine the respective virtues of the cloud chamber and the emulsion? In May, 1952, I began to try a new approach to the problem, and I soon decided to explore the possibility of a liquid medium.