July 17, 1919] 



NATURE 



399 



diameter to wave-length do not affect the results. 

 Rain, cloud, and fog are formed of such drops. The 

 opacity of a space containing a number of drops in- 

 sufficient completely to obliterate objects on the far 

 side depends on the lowering of the contrast between 

 light and shade brought about by the light scattered 

 by them, and not on any blurring or lack of defini- 

 tion. The amount of direct light which reaches the 

 eye from a source within a fog or shower is pro- 

 portional to 2-^, where ml is the distance of the 

 source from the eye, and I is the thickness of the 

 stratum which reduces the direct light by one-half. 

 The reduction to one-half will be caused by such a 

 number of drops as would, if placed side by side in 

 a plane to which the ray is normal, cut off all the 

 direct light; but when the same number of drops are 

 distributed at random in a volume of thickness I in 

 the direction of the ray, they allow half the direct 

 light to pass, in consequence of the probability that 

 some of them screen others, and thus leave space for 

 direct radiation. A relation is shown between the 

 rate of rainfall (i in. per day= 1/86,000 in. per second) 

 and the opacity of a shower. 



Physical Society, June 13. — Prof. C. H. Lees, presi- 

 dent, in the chair.— Dr. Balth. van der Pol, jun. : 



Comparison of the wave-form of the telephone cur- 

 rent produced by a thermal detector and by a rectifier 

 in heterodyne reception. — Profs. E. Wilson and E. F. 

 Herroun : The magnetic properties of varieties of 

 magnetite. The magnetic properties of certain varie- 

 ties of magnetite as exhibited by crystallised, com- 

 pact, or massive specimens and detached particles 

 have been examined. In each case the susceptibility 

 has been found to vary with the magnitude of the 

 magnetising force after the manner of iron, the rela- 

 tive variation being much more pronounced in the 

 case of those specimens having the higher suscepti- 

 bilitv; The maximum susceptibility in the specimens 

 examined occurs at a force ranging from 13 C.G.S. 

 units in the crystal to 368, its magnitude varying 

 from 3-12 to 0127 C.G.S. units. The effect of heating 

 has been greatly to increase susceptibility in some 

 cases, and in others a negative effect has been pro- 

 duced. In the case of a specimen of Penrvn mag- 

 netite, the large increase in the susceptibility was 

 traced to the conversion of ferrous carbonate and 

 ferric oxide into magnetite. Very high susceptibility 

 in magnetite is never associated with high coercive 

 force or retained magnetisation, the greatest values 

 for the latter exhibited by specimens having an inter- 

 mediate value of susceptibility of the order of 03 or 

 0-4. Lower susceptibility may be associated with high 

 coercive force, but naturally the retained magnetisa- 

 tion is not verv great, owing to the lower maximum 

 of induced magnetisation. 



Geological Society, June 25.— Mr. G. W. Lamplugh, 

 president, in the chair.— A. E. KItson : Outlines of 

 the geology of Southern Nigeria (British West Africa), 

 with especial reference to the Tertiary deposits. The 

 oldest rocks in Southern Nigeria comprise a series of 

 quartzites, schists of various kinds, blue and white 

 marble, grey limestones, altered tuffs and lavas, amphi- 

 bolites, and gneisses. They may be classed provisionally 

 as pre-Cambrian. So far as they have been observed, 

 there is a great hiatus between the pre-Cambrian and 

 the next known sediments, the Upper Cretaceous. 

 Normally, these are slightly inclined rocks. Flanking 

 the Udi plateau on the south and south-east, and 

 extending thence over the southern part of the great 

 valley to the Cross River, is a series of Eocene 

 estuarine shales, clays, and marls, with septarian 

 nodules and pieces of coal and resin, and a rich fauna 

 consisting principally of mollusca, but including frag- 



NO. 2594, VOL. 103] 



mentary remains of whales, birds, fishes, and turtles. 

 A thick series of sandstones, mudstones, shales, and 

 seams of brown coal forms a large portion of the 

 basin of the Niger, west of the Udi plateau. In the 

 Ijebu Jebu district are bituminiferous sands and clays 

 with Pliocene estuarine shells. Extending over prac- 

 tically the whole of the country south of lat. 7° 10' N., 

 and west of the great valley of the marine Cretaceous, 

 is a varying thickness of (usually unstratified) clayey 

 sands, probably late Pliocene — the Benin Sands series 

 of Mr. J. Parkinson. Along the coast-line and ex- 

 tending for considerable distances up the Niger and 

 Cross Rivers are fluviatile, deltaic, littoral, and swamp 

 gravels, sands, and muds of Pleistocene and Recent 

 age. In the Cross River basin, intruded into the 

 marine Cretaceous, are volcanic necks of decomposed 

 agglomerate, and sills (?) and dykes of olivine- 

 dolerite. These are probably pre-Eocene. The 

 Yorubaland crystalline rocks contain magnetite in 

 considerable quantities, while these and the crystal- 

 line rocks of the Oban Hills show smaller quantities 

 of cassiterite, gold, monazite, and columbite. — J. B. 

 Harrison and C. B. W. Anderson : Notes on the 

 extraneous minerals in the coral-limestones of Bar- 

 bados. Characteristic representative specimens of the 

 fossil reef-corals and of the beach-rock of the high- 

 level and low-level limestone terraces of Barbados 

 were examined chemically and microscopically in 

 order to ascertain the composition, nature, and origin 

 of their extraneous mineral contents. Chemical 

 analyses of the residua were made, and' the results 

 of these and of the microscopical examinations are 

 tabulated in the paper. The extraneous minerals 

 present were found to be apparently fresh and largely 

 unaltered fragments of wind-borne volcanic minerals 

 and glass. It was found that the volcanic minerals 

 enclosed in the reef-corals on which they fell have 

 been protected from change; those in the clastic 

 limestone or bed-rock show signs of detrition and 

 weathering prior to the consolidation of the lime- 

 stone. Similar minerals separated from clay norrnally 

 formed and accumulated in a pothole in the lime- 

 stone supply evidence of weathering changes after 

 being set free from the rock. It is shown that the 

 composition of the sedentarv residual soils on the 

 higher limestone-terraces of Barbados corresponds in 

 its essential parts with the residua separated, either 

 naturallv or artificially, from the limestone. The pro- 

 portions" of magnesium carbonate present in the coral- 

 rock are briefiv discussed, and complete analyses of 

 the high-level and the low-level limestones are given. 



Dublin. 

 Royal Irish Academy, June 23.— The Most Rev. J. H. 

 Bernard, president, in the chair.— A. Henry and Miss 

 M G. Flood : The history of the Dunkeld hybncj. 

 larch Larix eurolepis. This tree is raised in large 

 quantities from the seed of ten Japanese larches (L. 

 lebtolepis) growing at Dunkeld in the vicinity of 

 numerous European larches (L. europaea), from which 

 pollen is wafted bv the wind. The seedlings are 

 intermediate between the two parents, as shown by 

 microscopical examination of the sections of the 

 leaves, bv the colour and form of the bracts and 

 scales' of 'the cones, and bv the colour of the twigs, 

 leaves etc. The hvhnd seedlings, of which more than 

 100 acres have been planted on the Dunkeld, Athol, 

 and Murthlv estates, are very vigorous. Attention 

 is directed to the function of the papillae on the surface 

 of the leaf, which are constant in L. leptoUpts absent 

 in L europaea. and only present on a few cells in the 

 case of the hybrid. Reference is also made to other 

 hybrid conifers, including L. marschhnsn, Coaz, 

 which has recently appeared in Switzerland; L. pen- 



