Three Papers Concerning Experiments

Recent developments in spectroscopic techniques allow for accurate measurement of orientations of structural units in deformed polymeric systems Infrared dichroismCI-R-D) H - N-M.R. spectroscopy, fluorescence polarization (F.P.) are three methods which find increasing use in experimental investigations- The experiments reported in the present paper concern effectively simultaneous measurements of orientation by fluorescence polarization and of stress-strain relationship. It is worth noting that the transition moments of the label involved in fluorescence polarization are not sensitive to their immediate environment. Previous models are briefly reviewed and a new model of orientation of real networks is proposed- Then the fluorescence polarization technique is described and theoretical predictions are compared with measurements on polyisoprene vulcaniza-tes. 

The use of computer simulations to study internal motions and thermodynamic properties is receiving increased attention. One important use of the method is to provide a more fundamental understanding of the molecular information contained in various kinds of experiments on these complex systems. In the first part of this paper we review recent work in our laboratory concerned with the use of computer simulations for the interpretation of experimental probes of molecular structure and dynamics of proteins and nucleic acids. The interplay between computer simulations and three experimental techniques is emphasized (1) nuclear magnetic resonance relaxation spectroscopy, (2) refinement of macro-molecular x-ray structures, and (3) vibrational spectroscopy. The treatment of solvent effects in biopolymer simulations is a difficult problem. It is not possible to study systematically the effect of solvent conditions, e.g. added salt concentration, on biopolymer properties by means of simulations alone. In the last part of the paper we review a more analytical approach we have developed to study polyelectrolyte properties of solvated biopolymers. The results are compared with computer simulations. 

The following month, on April 22, Joliot, von Halban and Kowarski published a second paper in Nature concerning secondary neutrons. This one, Number of neutrons liberated in the nuclear fission of uranium, rang bells. Calculating on the basis of the experiment previously reported, the French team found 3.5 secondary neutrons per fissioa The interest of the phenomenon discussed here as a means of producing a chain of nuclear reactions, the three men wrote, was already mentioned in our previous letter. Now they concluded that if a sufficient amount of uranium were immersed in a suitable moderator, the fission chain will perpetuate itself and break up only after reaching the walls limiting the medium. Our experimental results show that this condition will most probably be satisfied. That is, uranium would most probably chain-react. 

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