No abstract
LETTERS TO THE EDITOR68i the lead of the precipitate and not in the thallium of the filtrate. Neither of the activities was observed following bombardments of mercury targets. Thresholds for production were compared with the threshold for production of 8-hour Pb 201 , and this comparison supported the proposed assignments. Comparison of the energy of the 5.6-second 7-ray with the energy of a 7-ray found in polonium extracted from a bismuth target indicated that the a-decay of Po 206 probably goes to the 5.6-second excited level of Pb 202 .Intensity of the 0.67-Mev line of the 50-second activity appeared to be about four times that of the two lower energy lines on the basis of comparisons of the areas under the photoelectric capture peaks. Although definite transition types cannot be specified, it is interesting to note that the energies 0.42 and 0.25 Mev are in line with the energy spacings of iw 2 to / B / 2 and / 5 / 2 to pm levels for adjacent isomers of odd neutron number as plotted by Hill. 3 The type of transition giving rise to the intense 0.67-Mev line is particularly puzzling since a crossover transition 213/2 to pi/2 is ruled out as being of type M6. Further data are required before this point can be clarified.In the course of this investigation the period of the known Pb 207 metastable state was redetermined as 0.80±0.02 second.T HE fact that below 1 °K the speed by which heat pulses travel in He II increases enormously 1 has generally been regarded as a confirmation of the Landau version of the two-fluid model. 2 According to that version the so-called normal fluid would essentially consist of rotons, the role of which would be taken over by phonons below 1°K.In different laboratories it has been observed, however, that the width of the heat pulses increases greatly, and this might suggest that at the lowest temperatures one does not have to do with second sound proper but with the normal transmission of heat pulses in a medium of a certain specific heat and thermal conductivity. The formulas of Kronig and Thellung 3 might be appropriate to describe the transition to the phenomenon of second sound above 1 °K. The magnitude of the speeds observed suggests that the mean free path of the phonons responsible for the thermal conductivity is determined by the width of the vessel. 4 The difference with Landau's picture would consist of his emphasis on the momentum of the phonons and of their large effective diameter for collisions.Mr. H. C. Kramers in Leiden is preparing to investigate the expected influence of the length and the width of the vessel. The publication of the present letter was stimulated by a discussion with Dr. Pellam and Dr. De Klerk, who kindly showed me the recent results obtained at the Bureau of Standards.Finally, it might be suggested that the increase of the second sound velocity 5 above 1°K upon dissolving He 3 also could be accompanied by an increase of damping and dispersion and so indicate an earlier transition to the very low temperature case. T HE 7-hr isomer activity in molybde...
EDITOR
459The initial condition on H is that it be uniform throughout all space. The boundary condition is that H have a value Ho independent of time on the surface of the cylinder (r-ro). The solution of Eq. (1) subject to these conditions isxfj exp(a n 2 u)fy(u)-l]rf«|. (2) Here, we have R~r/c\ 0 *, T**/
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