Number 178 (Story #1), May 10, 1994 by Phillip F. Schewe and Ben Stein THE LATTICE SPACING IN SILICON (220) has been measured to be 192015.569 fm (1 fm is 10**-15 m) with a relative uncertainty of 3 x 10**-8. Because silicon technology can provide the largest pure crystals with well-characterized dimensions (the <220> notation refers to the particular direction through which the crystal is viewed) a careful measurement of the spacing between atoms can facilitate a count of the atoms in the crystal. This in turn can be used to calculate a better value for the Avogadro constant, equal to the number of atoms in a mole. Scientists at the University of Torino in Italy, employing a combination of x-ray and optical interferometry to measure the lattice spacing, believe their work will also lead to a better determination of the wavelengths for thermal neutrons and for x rays and gamma rays. (G. Basile et al., Physical Review Letters, 16 May 1994.)