240 



NA TURE 



[August 18, 1923 



Evidence of cross-bedding, which is inseparable from 

 this type of rock, would be easily lost, since the 

 grey wethers axe secondarily silicined or " concre- 

 tionary." From conversation with the late Prof. 

 Rupert Jones, than whom 1 knew no keener observer, 

 I gathered that he firmly believed in the rootlet 

 and stem structure of these perforations (see Geol. 

 Mag., 1901, pp. 54-59 and 115-125). Another re- 

 corded instance of enclosed rootlets is^ given by 

 Wm. Carruthers (Geo/. Mag., 1885, p. 361); who found 

 in a weathered sarsen stone from Abury a root with 

 rootlets, which he doubtfully ascribes to a palm, and 

 in the position of growth. 



It would be interesting to discover any positive 

 evidence of cross - bedding in these white Tertiary 

 sandstones. The Bagshot sands, by the way, both 

 in Surrey and Kent, are often strikingly and steeply 

 cross-bedded, and this, from a study of our dune 

 rock in Victoria, points to aeolian formation rather 

 than to marine current action. Fredk. Chapman. 



National Museum, Melbourne, 

 June 15. 



Barometric Pressure in High Latitudes. 



I AM much obliged to Mr. L. C. W. Bonacina 

 (Nature, July 21, p. 100) for pointing out a clerical 

 error in my statement concerning the winter and 

 summer Arctic pressures. The correction gives 

 greater emphasis to my contentions. 



My point is that in the Arctic regions, even during 

 the winter when the sun's light does not reach the 

 area to any"^extent, the pressure is low, indicating 

 a sufficiently warm stratosphere able more than to 

 counterbalance the effect of the cold lower tropo- 

 sphere. 



The lower troposphere over the polar areas is 

 undoubtedly very cold, and this cold air often flows 

 outwards from the poles for some distance. I am 

 not aware that my views on this point are in conflict 

 in any way with those of Dr. G. C. Simpson, Prof. 

 Mohn, or Prof. Bjerknes, except on very minor 

 points. What I have attempted to explain is not 

 why these northerly Arctic winds exist, but rather 

 why they do not blow from the poles to the equator. 

 The real difficulty, to my mind, is to account for the 

 westerly poleward winds of middle latitudes. 



Mr. Bonacina says " there must, on the average, 

 be a relatively high surface pressure about the poles." 

 But all the charts show a relatively low pressure. 

 However, an outflow of cold air from the poles will 

 occur if the density of the lower troposphere decreases 

 with sufficient rapidity as we move towards lower 

 latitudes ; and this is what actually often occurs, for the 

 temperature rises as we move from the poles. 



R. M. Deeley. 



Tintagil, Kew Gardens Road, 



Kew, Surrey, 



July 20. 



Phototropic Compounds of Mercury. 



In Nature of June 9, p. 775, Messrs. Venkatara- 

 maiah and Rao describe " A New Phototropic Com- 



X 



HS 



pound of Mercury " of the composition Hg 



\ 



CNS 



which they regard as "the most phototropic com- 

 pound as yet known " ; or that this compound 

 shows appreciable change in colour on exposure to 

 light in less time than that required by any other 

 known phototropic compound. In 1917, while work- 

 ing in the College of Science, Calcutta, in an attempt 



NO. 2807, VOL. I 12] 



to prepare (SHgl)„ described by Kay (Trans. Chem. 

 Soc., Ill, log), without using any organic compound, 

 I obtained 2HgS . Hgl,, which showed phototropy to 

 a remarkable degree. The orange yellow powder 

 turned black very quickly on exposure to sunlij^ht, 

 but only gradually in diffused daylight. On keeping 

 the black powder in the dark, the reverse change took 

 place. At room temperature, it took several hours 

 to recover, but at higher temperatures the change of 

 colour was quicker ; at about 85° C, for example, 

 only a few seconds. Both varieties had the same 

 chemical composition. This substance was exhibited 

 before the Indian Science Convention of that year, 

 and a preliminary note was published in the Re|X>rt of 

 the Indian Association for the Cultivation of Science, 

 191 7. Since then I have found that phototropy is 

 exhibited more or less by all the complex sulphides 

 of mercury of the general formula HgS . HgX, or 

 2HgS . HgXj, where X is a halogen or a monovalent 

 acid radicle, including CNS, of which aHgS . Hgl, is 

 the most sensitive. 



The sensitiveness to light depends to some extent, 

 as might be expected, on the nature and area of the 

 surface exposed. I have found that paper coated 

 with an emulsion of 2HgS.Hgl2 in gelatin is much 

 more sensitive to light than the powder. In fact, it 

 turns black more quickly on exposure to light than 

 the ordinary gelatino-chloride paper used in photo- 

 graphy. But it is very curious that in this case the 

 reverse change of colour does not take place on 

 keeping in the dark or heating. Evidently the gela- 

 tin somehow prevents the reversal. A detailed 

 report on these inorganic phototropic compounds will 

 be published in due course. M. L. I>kv. 



Central Chemical Laboratory 

 Kirkee, India, July 5. 



Melanism in the Lepidoptera and its 

 Possible Induction. 



Believing that light can be thrown on some of the 

 problems of evolution by an experimental investiga- 

 tion of the development of melanism in lepidoptera, 

 we have been studying the influence of the food 

 plants growing in critical areas, and also of inorganic 

 substances likely to occur in or on the plants of such 

 regions, on races of moths imported from non- 

 melanic districts. Our cultures have been reared at 

 two centres ; some at Birtley (Durham), an area 

 producing a very large number of melanic sp)ecies, 

 and others at Hexham (Northumberland), where 

 melanism is much less prevalent, although not absent. 

 The work is not finished, but certain facts seem 

 worth publishing at once, particularly in view of the 

 recent controversy as to the value of Kammerer's 

 experiments. 



We began wuth Kentish races of Tephrosia crepus- 

 cularia Hb., and Kent and Hampshire strains of 

 T. histortata Goeze, rearing them on hawthorn 

 gathered by the roadside at Birtley, and in the third 

 generation of T. crepiiscularia, a species in which we 

 have proved melanism to be a Mendelian dominant, 

 obtained one black female in a brood of 23 insects. 

 T. histortata, on the other hand, showed no change 

 in the fourth generation, at which stage the eggs 

 from one pairing were sent to Hexham and others 

 reared at Birtley, where in the next (fifth) generation 

 one black female was obtained from about 90 pupae. 

 The eggs at Hexham, cousins to those at Birtley, 

 were divided into four batches, the larvae in one 

 case being fed on local hawthorn and in the others 

 on hawthorn impregnated with a metallic salt. In 

 each culture one or two black moths appeared, the 

 broods averaging two dozen in number. 



