A kinetic study of the alkaline transition of DNA, in clearly defined physico-chemical conditions, is presented, which allows us to identify, within the alkaline transition region, different pH ranges, corresponding to different ratelimiting factors. This analysis brings into consideration three distinct intervals of time which characterize the whole process, namely the time necessary for full hyperchromicity to be reached, the time required for strand separation in the case of a single DNA molecule, and the time for complete denaturation to be reached in the case of a DNA solution.THE RESULTS OBTAINED FROM ULTRACENTRIFUGAL, AND SPECTROPHOTOMETRIC MEASUREMENTS, INVOLVING RAPID MIXING EXPERIMENTS, SEEM TO INDICATE THE FOLLOWING CONCLUSIONS: whereas, in the lower pH ranges considered within the transition region, the denaturation process is limited by the first time constant, this same constant becomes extremely short at higher pH. On the other hand the fact that, in the higher pH range, the second and third time constants do not coincide (the time to unwind a single T2 DNA molecule being at least one order of magnitude shorter than the time required for bulk denaturation to be reached) suggests that in this pH range the overall denaturation rate is limited by a statistical process governing the initiation of unwinding.These observations are discussed in terms of a model in which the unwinding energy is given by the electrostatic repulsions which originate in the deprotonated DNA molecule. The model itself suggests some experiment which seem to confirm it.

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The fission cross section of uranium was measured for neutrons produced in the following reactions: Rn+Be, P+C, D+D, D+Be, P+B, D-f-Li. From the fact that the mean cross section has about the same value for neutrons of the D + D, D-f-Be, Z?+B reactions, it was concluded that the fission cross section has a value

The scattering cross sections of the proton, deuteron, and the carbon nucleus have been determined for 4.1-, 12.5-, and 13.5-Mev neutrons by detecting the corresponding recoil protons. A coincidence set of three proportional counters was used. Similar measurements were made with 14-Mev neutrons from the reactions D+Li. These were detected by the 9.9-min. activity arising from the reaction 29Cu 63 +oft 1 -*-29Cu 62 +2oW x . The n -2n disintegration of the deuteron has been investigated for 14-Mev neutrons and cross section found to be of the order of 9-4 X10~2 6 cm 2 ..The scattering cross sections at 4.1, 12.5, and 13.5 Mev are, respectively, for H 1.73, 0.69, 0.694X10-24 cm 2 ; for D 1.79, 0.78, 0.864X10" 24 cm 2 ; for C 1.99, 1.40, 1.23X10" 24 cm 2 . The experimental results are compared with those of other experimenters and with theoretical predictions.