922 NATURE December II, 1948 Vol. 162 nuclei' should be reserved for any which permit A Concentration of Uranium in sublimation from vapour to ice strictly at the Black Muds frost-point. The ice crystals formed in outdoor air do not, on IN the Cambrian black shales of the Oslo area, evaporation, leave behind any sublimation nuclei. If there is a considerable concentration of uranium, the temperature of the chamber be raised gradually, analyses values ranging from 25 to 180 gm.Jton. clouds again appear mainly as water droplets as (These and the following uranium analyses were made soon as final temperatures fail to reach - 41° C. at theNorwegian Defence Research Institution, which Cleaned air. Experiments were made in air from has given permission for their publication.) which the great majority of nuclei ha d been removed An interesting problem is how far a concentration by filtration. Down to - 55° C. and with expansions is going on in the present-day black muds formed of ratio up to I : 10 (the limits obtainable), con• under anaerobic conditions. There is a large number densation or freezing occurred only on the few nuclei of land-locked waters around the Norwegian coast, remaining. Irradiation of the chamber with gamma in which these black muds are deposited1• rays from a radioactive source produced no effect. During the summer of 1947, Ire-investigated these This is in contradiction to the observation of fjords, and the uranium content of some mud samples Cwilong1• was determined. All analyses are made by the ether• Contaminated air. The threshold at - 41° C. has extraction method and are thus independent of the been observed not only in outdoor air but also in radium content. In such recent sediments uranium cleaned air contaminated with a wide variety of and radium are not in an equilibrium. artificial nuclei, including those from sprayed sodium The localities have already been described\ and chloride and sodium nitrate solutions and from a are here arranged according to the quantities of red-hot platinum wire. uranium found. The depth is also stated, as well as Certain nuclei, however, have been found to the content of hydrogen sulphide and oxygen in the stimulate ice formation at higher temperatures. Silver deep waters. iodide, the effect of which on supercooled water fogs 1 has been reported by Vonnegut , provides, when Depth from Maximum hydro- volatilized, freezing nuclei active at temperatures which sample Content of gen sulphide or as high as - 6° C. I.ocallty is collected uranium minimum oxygen (m.) (gm./ton) in deep waters Only one instance of true sublimation has yet (cm.'/1.) been observed. This was on nuclei produced by a Topdalsfjord 65 60 c. 1 H,S spray of cadmium iodide solution, but only after Sondeledpoll 18 50 5·4 an ice cloud had been formed on them by expansion IsefJ::erfjord 26 50 7·9 Hellefjord 70 40 40·7 to a temperature below - 42° C. The ice crystals Framvaren I 160 40 199·5 thus formed left behind, on evaporation, small Framvaren II 160 35 199·5 Vestrhusfjord 53 35 9 ·9 particles which acted as true sublimation nuclei at Mortensfjord 34 20 1·6 temperatures up to - 10° C. Further warming to Drammens- fjord 117 15 4 ·8 - 9° C. caused a 'melting' of these nuclei, which Oksefjord 81 13 3 ·2 o, then reverted to giving only water droplets, even on I I subsequent cooling to - 25° C. It may be noted that the freezing point of a saturated solution of There seems to be a definite connexion between cadmium iodide, in bulk, is in the neighbourhood drainage from granite areas and uranium content in of- 9° C. the black muds, this being most conspicuous with Both silver iodide and cadmium iodide occur in the record value of 60 gm.Jton in Topdalsfjord, into hexagonal crystals which have lattice constants within which empties a large river draining extended 7 per cent of those of ice-!. granite areas. Nuclei from rain-water. Some recent experiments The precipitation probably starts by the formation on the nuclei produced by a spray of rain-water of sulphide on the interface between the upper collected during a thunderstorm have revealed the ventilated brackish waters and the salt waters of the presence of small numbers of nuclei which gave deep, containing hydrogen sulphide. ice crystals at temperatures as high as - 20° C., but By comparing my old core samples with echo which were evidently not common enough to be soundings taken during the summer of 1948, I was detected by normal experiments on outdoor air. able to ascertain the thickness of the layers of blackish These may be the nuclei discovered by Findeisen3 , muds to range from less than a metre up to some though it was clear that they acted as freezing, and not 20 metres in a few isolated places. sublimation, nuclei. Two of the three fjords which stand apart by their My thanks ax:e due to Lord Cherwell for enabling low values are, as a rule, quite well ventilated, while me to work at the Clarendon Laboratory ; and to the in the Drammensfjord the muds are not very black Department of Scientific and Industrial Research and there is probably a considerable redeposition of for a maintenance grant. It is a pleasure to acknow• clay from the shores. Even here there is some con• ledge my debt to Prof. G. M. B. Dobson for his centration in relation to the granitic batholith in continued interest and valued advice on many which the Drammensfjord is situated ; but from which occasions. it receives only a very small part of its freshwater E. M. FoURNIER n'ALBE supply, and in which the uranium content is some Clarendon Laboratory, 10 gm.Jton. Oxford. KAARE MUNSTER STR0M June 30. Department of Limnology, University of Oslo, 1 Cwilong, B. M., Proc. Roy. Soc., A, 190, 137 (1947). Blindern, Norway. • Vonnegut, B., J • .App. Phys., 18, 593 (1947). Aug. 10. • Findeisen, W. and Schulz, G., "Forschungs und Ehrfahrungsberichte des Reichwetterdienstes", A, No. 27 (1944 ). • Strem, K. M., Skr. VidMik.·A.kad. Oslo, 1, 7 (1936)

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