164 The Poetry of Physics and The Physics of Poetry
violent is the motion of the grain. Einstein pointed out in 1905 that this
jerky motion is due to the fact that the pollen grain is being constantly
bombarded by water molecules. From his detailed calculations of the
pollen grain’s motion, Einstein was able to determine the absolute weight
of various atoms and, hence also Avogadro’s number. His analysis of
Brownian motion actually provided the first direct detection of the atom.
Additional evidence for the composite nature of the atom was
gathered towards the end of the nineteenth century as new forms of
radiation were discovered. Roentgen discovered x-rays in 1895 when
working with his gas discharge tube, he noticed a glow in some
fluorescent material lying near his apparatus. These rays were produced
as a result of the bombardment of the atoms of the anode by electrons
being accelerated there by the electric field in the gas discharge tube.
As a result of the bombardment by the electrons, the atoms in the
anode were excited and, as a consequence, emitted x-rays. It was not
until the Van Laue experiments of 1912 that it was realized that
x-rays are electromagnetic radiation of extremely small wavelength. The
wavelengths of x-rays are of the order of 10-8 cm, the distance between
atoms, which explains their ability to penetrate solid matter. Van Laue
passed an x-ray beam through a crystal in which the atoms are arranged
in an orderly array. He found that the x-rays displayed the characteristic
diffraction pattern one observes for light passing through a system of
slits. The spaces between the atoms in the crystal served as the system of
slits. From the diffraction pattern and the knowledge of the spacing
between the atoms in the crystal, other investigators were able to
determine the wavelength of the x-ray. It was discovered that the
x-rays produced by a particular atom had certain discreet characteristic
frequencies analogous to the situation with the discreet lines of visible
light emitted by gas atoms.
Roentgen’s discovery of x-rays stimulated other researchers to look
for still different forms of radiation. In 1896, Becquerel began a series of
experiments with phosphorescent materials such as the uranium
compound, potassium uranyl sulfate, which glows in the dark after it is
exposed to sunlight. Becquerel was hoping to show that the radiation
from phosphorescent materials contained x-rays. Quite by accident, he
made another discovery, which turned out to be far more important.
Becquerel left a sample of the potassium uranyl sulfate in his desk
drawer with some photographic film and discovered, much to his
surprise, that, although no light had fallen upon the film, it was still