98 



NATURE 



[March 29, 19 17 



also to the basalts of the whole Arctic region stretching 

 from Dickson Harbour to West Greenland. The 

 essential minerals are labradorite, rich in the anorthite 

 molecule, pyroxene of the onstatite-augite type, and 

 titaniferous magnetite. The province as a whole dis- 

 plays significant variations both in time and space. 

 The earliest eruptions are generally poor in alkalies, 

 but tend to become more alkaline as the present period 

 is approached. Thus, the later eruptions of Spits- 

 bergen gave rise to olivine trachydiorites instead of 

 basalt. Jan Mayen still possesses an active volcano, 

 and its rocks are unusually alkaline basalts. Similarly, 

 the later rocks of Iceland and, to a lesser extent, of 

 Skye and the Small Isles follow the same course. In 

 space the most remarkable variation is seen in the 

 distribution of titanium, the percentages of titanium 

 oxide being high in the rocks of Greenland and the 

 Iceland Ridge, and falling away regularly on each 

 side. The Brito-Arctic Petrographic Province can be 

 subdivided into five regions, viz. the British, the Ice- 

 landic (including the Faroe Islands and the Scoresby 

 Sound district), the West Greenland, the Jan Mayen, 

 and the Spitsbergen — Franz-Joseph Land — Dickson 

 Harbour, and the differences subsisting between them 

 nre related to the processes whereby the igneous 

 activitv was initiated. It is suggested that a petro- 

 graphic province consists of a number of adjacent 

 regions of igneous activity, in which similar rocks, 

 or similar series of rocks, have been produced, whence 

 It follows that the processes by which the magmas 

 have been formed, differentiated, and intruded must 

 be similar, and the underlying materials on which 

 these processes have acted must also be similar. — Dr. 

 J. W. Evans : A general proof of the limitation of 

 the symmetry-numbers of crvstals. On the assumption 

 that crystals are composed of cells identical in all 

 respects, then, if n be the degree of the symmetry 

 of an axis and d an integer, the equation 



cos — = A(i-^/) 

 ;/ 



must be satisfied. The only possible values of d are 

 3, 2, I, o, the correspondingf values of n being 

 2, 3, 4, 6. — E. S. Fedorov : The numerical relation 

 between zones and faces of a polyhedron. The numerical 

 relation shown^ by axes of symmetrv situated in planes 

 of symmetry pointed out by G. Cesaro in 1Q15 is only a 

 particular case of the more general one deduced bv the 

 nuthor in 1885.— A. Lcdoux, T. L. Walker, and A. C. 

 "Wheatley : The crystallisation of parahooeite. Crystals 

 in the Royal Ontario Museum of Mineralogv from 

 the original locality. Broken Hill, North-Western 

 Rhodesia, are triclinic with the axial ratios a'.h : c = 



o'772q : I : 07124; = 93"" 22', /3 = ot' 



7 = qi 22 



Thirty-two forms are recorded. The crystals have 

 perfect cleavage parallel to the brachypinacoid, and 

 show lamellar twinning parallel to the macropinacoid. 

 The anp'le of optical extinction on the cleavage is 10° 

 with reference to the twin-lamellae. 



Royal Meteorological Society, March 21.— Major H. G. 

 Lvons, president, in the chair.— Major G. I. Taylor : 

 The formation of fog and mist. Fogs are due either 

 to precipitation of water in the air or to a condition 

 of the atmosphere which prevents smoke from being 

 dispersed from the air close over the roofs of a town. 

 The two necessary conditions for the formation of a 

 smoke fog are that the w-ind velocity must be very 

 small and the air near the ground must be relatively 

 cold compared with the air higher up for a period 

 sufficiently long to collect enough smoke to form a 

 fog. The formation of fog at sea can usuallv be 

 traced to the cooling of the surface air when it flows 

 from a place where the sea is warm to a place where 

 it is cold, but sometimes a fog is caused by air flow- 

 ing from a cold to a warm part of the sea. In the 



NO. 2474, VOL. 99] 



former case the fogs are usuaUy low-lying and thick, 

 while in the latter they are more frequently light fogs 

 w'hich stretch up to a considerable height. Fogs con- 

 sisting of small drops of water are formed on land, 

 too, by the cooling of surface air, but in this case 

 the air usually stays still, while the lowering of the 

 temperature of the ground by radiation to the sky at 

 night cools the air near the surface. Fogs of this 

 type are not formed until the temperature has fallen 

 considerably below the dew-point of the air during 

 the day. This is because the formation of dew dries 

 the air near the ground; Theoretical considerations 

 show that the amount by which the temperature must 

 fall below the dew-point before fog is produced depends 

 on a complicated series of causes, but an empirical 

 method has been devised for estimating whether, on 

 any given night, there is enough water vapour in the 

 air to form a fog if other conditions are suitable. This 

 method can be used for local forecasting. 



CAMBRIDGE. 



Philosophical Society, February ig. — Dr. Marr, presi- 

 dent, in the chair.— B. Sahni : i. An Australian speci- 

 men of Clepsydropsis. 2. Observations on the evo- 

 lution of branching in ferns. The evolution of the 

 branching of the fern stem is discussed for the first 

 time from the point of view of vascular anatomy. It 

 is concluded that dichotomous branching is primitive 

 and that monopodial branching is derived from it by 

 the successive intercalation, at the base, of a series 

 of stages, each morphologically less complex than the 

 preceding. The process has thus been one of retro- 

 gressive evolution in the basipetal direction. — C. P. 

 Dutt : Some anatomical characters of coniferous wood 

 and their value in classification. The author directs 

 attention to the confusion in existing accounts of the 

 pitting associated with medullary ray cells and gives 

 the result of an investigation on the same subject. 

 Conclusions are drawn as to the value of such pitting 

 as a diagnostic character. 



M.-VNCIIESTER. 



Literary and Philosophical Society, February 20. — Prof. 

 S. J. Hickson, president, in the chair. — Dr. W. 

 Makower : The photographic action of a rays. The 

 first important investigation of the photographic action 

 of o particles was made in 19 10 by Kinoshita, who 

 succeeded in showing that whenever an a particle 

 strikes a grain of silver ha'ioid in a photographic plate, 

 that grain is af:erwards capable of photographic 

 development ; moreover, this was true throughout the 

 range of the a particle. Later it was shown by 

 Reinganum and others that when a particles are pro- 

 jected tangentially to a photographic plate, after de- 

 velopment the film shows definite trails of grains of 

 silver halide, which can readily be distinguished under 

 the microscope. These trails are produced by the 

 impact of the. o particles on the haloid grains_as they 

 pass through the film, and their length represents the 

 range of the a particles in the film of gelatine. Photo- 

 micrographs showing the paths of a particles through 

 photographic films were first published by Walmsley 

 and Makower, and soon afterwards by Kinoshita and 

 Ikeuti. The method adopted bv the latter was to 

 activate the tip of a sewing-needle by gently rubbing 

 it on a surface coated with .the active deposit of 

 radium or some other source of a radiation. In this 

 wav a trace of active matter was transferred to the 

 point of the needle, which was then placed for a short 

 time in contact with a photographic film. The grains 

 affected by the a particles can be clearly seen radiating 

 out in straight lines from centres representing the 

 points at which the needle had been brought into 

 contact with the films. 



