The positions of the five crystallographically independent hydrogen atoms in the mineral brushite, CaH P04,2H,0, which crystallizes with Z = 4 in the non-centrosymmetric monoclinic space group l a (c), have been established by neutron-diffraction analysis based on 632 independent reflexions from synthetic crystals. Anisotropic threedimensional least-squares refinement of all the atoms in the structure to R 0.033 gives final estimated standard deviations of 0.002 for calcium ions, ca. 0.006 for phosphorus and oxygen atoms, and 0.01 0-0.01 5 A for hydrogen B. Dickens, in ' Symposium (1969) on Structural Properties of Hydroxyapatite and Related Compounds,' ed. W. E. Brown, Gordon and Breach, New York, 1971.

A study by neutron diffraction of crystals of solid benzene at — 55 and — 135 °C has extended the earlier picture of the molecular structure provided by Cox’s X-ray work at — 3 °C. Neutron reflexions in four crystal zones have been examined, leading to values of the atomic co-ordinates and thermal parameters. Superimposed upon the random motion of the carbon and hydrogen atoms, there is an oscillatory angular motion of the molecule in its own plane for which the root-mean-square amplitudes are 7.9°, 4.9° and 2.5°, respectively, at the three different temperatures. At the same time there are small changes in the orientation of the molecules as the temperature is reduced, of such a kind that they pack together more easily along the Z axis, thus accounting for the enhanced thermal expansion along Z compared with X .

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Such a picture is best seen with a stereoscopic viewer.After a little practice, however, many people can achieve stereopsis without instrumental aid. The picture should be viewed in a good, even light; it helps if a piece of card is held between the eyes, and normal to the picture, so that each eye can see only its appropriate half of the diagram.

SchlussbemerkungWelehe Methoden man zur genauen Schweratomparameterbestimmung heranziehen wird, hi~ngt vom Problem ab. Im Falle yon Eiweisskristallen ist, wie die Betrachtung der Korrelationsfunktionen gezeigt hat, der quasistatistisehe Zusammenhang der Reflexe so ausgepr/igt, dass die gewichtete C'-oder C"-Funktion bereits gute Ni~herungswerte liefern sollte, die nachtri~gliCh noch verfeinert werden kSnnen. In niederatomigeren azentrischen Kristallen kann allerdings die u+-Funktion bessere Resultate liefern. Von den Verfeinerungsmethoden scheint uns die Differenzfouriersynthese die geeignetere zu sein, da sic nicht nur die Schweratomparameter zu verbessern gestattet, sondern auch die Gtite der gefundenen Struktur beurteilen 1/~sst. Allerdings ist die Rechnung nach der Methode der kleinsten Quadrate insbesondere im dreidimensionalen Fall schneller und einfacher, so dass es sich vielleicht empfiehlt, zur Abkfirzung der Rechenarbeit zuerst die beste LSsung nach der 3/Iethode der kleinsten Quadrate zu suchen und diese dann durch eine Differenzsynthese zu priifen. The methods of neutron diffraction, which permit a distinction to be made between carbon and nitrogen atoms, have been used to extend the earlier X-ray studies of K3Co(CN)6. It appears that the carbon atoms are in positions adjacent to the cobal¢ ions. The methods of Fourier synthesis and least squares have been used to obtain improved atomic co-ordinates and values for the DebyeWaller temperature factors. The length of the C-N bonds is 1.15 /~.