In 1912 another German physicist, Max von Laue, realized that X-rays were scattered and deflected at regular angles when they passed through a copper sulfate crystal, and so he produced the first X-ray diffraction pattern on a photographic film.A year later William Bragg of Britain and his son Lawrence perceived that such a pattern reflects the layers of atoms in the crystal structure, and they succeeded in determining for the first time the atomic crystal structure of the mineral halite (sodium chloride).
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Bertram Boltwood suggested that lead is one of the disintegration products of uranium, in which case the older a uranium-bearing mineral the greater should be its proportional part of lead.
Analyzing specimens whose relative geologic ages were known, Boltwood found that the ratio of lead to uranium did indeed increase with age.
These experiments are carried out at elevated temperatures and pressures that simulate those operating in different levels of the Earth’s crust.
Thus the metamorphic petrologist today can compare the minerals and mineral assemblages found in natural rocks with comparable examples produced in the laboratory, the pressure–temperature limits of which have been well defined by experimental petrology. Bridgman developed a technique for subjecting rock samples to high pressures similar to those deep in the Earth.
With all these deformation experiments, it is necessary to scale down as precisely as possible variables such as the time and velocity of the experiment and the viscosity and temperature of the material from the natural to the laboratory conditions.