The crystal structures of the hydrocarbons anthracene and tetracene have been investigated by X-ray diffraction methods. In the former compound a triple Fourier analysis was carried out, and the atomic co-ordinates have been determined with great accuracy. The observed variations in bond lengths within the molecule have been discussed in terms of the resonance structures, and a measure of agreement has been found to exist between the observed and calculated values. The effect of thermal vibration and the presence of hydrogen atoms have both been clearly indicated in the Fourier contour map drawn in the plane of the molecule. The preliminary investigation into the crystal structure of tetracene, by a double Fourier analysis, has led to a determination of the atomic parameters, and a close similarity to the anthracene structure has been revealed
