The importance of absolute, as opposed to relative, intensity measurements in X-ray crystal analysis has often been stressed, and such measurements may lead, with compounds where one sort of atom is of dominant importance, to a direct and unequivocal determination of the structure. Their value in the analysis of organic compounds, where usually no atoms are predominant in X-ray scattering power, is perhaps not so immediate; nevertheless, the absolute X-ray reflection of a given substance is at least as important a constant as, say, its thermal conductivity, and its determination is a necessary preliminary to any detailed study by the X-ray method. Hitherto such absolute measurements have been confined to inorganic substances
e. g.
, rock-salt, usually occurring in large crystals. It seemed to be worth while to make a determination of the absolute reflecting power of a typical organic crystal in conditions similar to those employed in the analysis of such crystals,
i. e.
, using a small crystal completely bathed in a monochromatic pencil of X-rays. A very suitable substance for such a measurement is
anthracene
, since it is easily obtained pure and in crystals which are reasonably permanent and simple to adjust, but more particularly because its structure is already known in considerable detain and relative intensity measurements described in this paper will enable workers with other crystals to standardize series of relative intensity measurements with the minimum of labour, by comparing directly one of their observed reflections with the (001) reflection from a small weighed crystal of anthracene. The comparison can be made without difficulty, either photographically using an integrating microphotometer or by the ionization method on an ordinary spectrometer.
The reflection of X-rays from anthracene crystals