100 YEARS AGO

It is known that salt (NaCl) at a temperature of 200° C is phosphorescent; during a course of experiments in June last I found that radium bromide induces phosphorescence at ordinary temperatures. The following is a convenient way of observing the phenomenon. Fill a wooden match-box with table salt removed from the inner portion of a block; press the radium bromide tube into the yielding mass and just barely cover it with the substance. If it be now put on one side for a few hours, say into one of the compartments of a chest of drawers, on opening the box in the dark all round the tube will be found to phosphoresce with a white light, but, unlike zinc blende and barium platinocyanide, the salt continues visibly to phosphoresce after removal of the radium bromide... The image of the visible portion round and where the radium bromide tube has lain is impressed on a photographic plate in thirty minutes, but only very faintly in two or three minutes.

From Nature 23 July 1903.

50 YEARS AGO

Evidence for 2-chain helix in crystalline structure of sodium deoxyribonucleate. Watson and Crick [Nature 171, 737, 1953] have proposed a structure for sodium deoxyribonucleate consisting of two co-axial helical chains related by a diad axis. We have shown [page 740 of the same issue] that the main features of their structure are consistent with certain important features of our X-ray diagrams of structure B (the high-humidity less-ordered form of the salt). A subsequent closer investigation of density and water content in relation to the prominent equatorial spacing, and also of equatorial intensities calculated from a projection of the proposed structure (kindly provided by Watson and Crick), makes it clear that in detail the structure is not consistent with the observed equatorial reflexions. Both density and intensity considerations lead us to favour a more compact helical structure in which the phosphorus atoms lie on a helix of radius about 8.5 A. rather than 10 A. This value also lies within the range of spread of the more diffuse layer-line peaks. We are more concerned here, however, with evidence which confirms in principle the type of structure suggested by Watson and Crick, than with criticism on points of detail.

Rosalind E. Franklin and R. G. Gosling

From Nature 25 July 1953.