X-Ray Studies of the Molecular Arrangement in Liquids

An improved technique has been developed for taking x-ray diffraction photographs of weakly absorbing liquids. Photographs of water, heptane, decane, benzene and cyclohexane have been taken and several hitherto unobserved details have been discovered. The photographs of water have been mathematically investigated by the method of Zernike and Prins, and it was concluded that nearly every molecule has four others around it. No evidence was found either for the definite ``quartz-like'' arrangement or for the extensive degree of close packing which are postulated by Bernal and Fowler. With the organic compounds there was apparent in every case a striking resemblance of the outer parts of the photographs to electron diffraction photographs of the corresponding vapors. This is in agreement with Debye's prediction and argues for a relatively low degree of periodicity in the liquid structure. It has been shown further that the two outer bands which occur in the photographs of the normal hydrocarbons should appear whenever the molecules of the liquid consist largely of lengths of straight saturated hydrocarbon chain. Some discussion of the structure of the organic liquids has been given. In particular, a theory on the structure of liquid benzene has been postulated and found, using the method of Zernike and Prins, to predict with satisfactory accuracy the observed photograph. The viewpoint which has been found useful in the present discussions is that, to a predominating extent, adjacent molecules in the liquid are held together in very nearly the same manner as in the crystal, while, except for this, the arrangement is random.