224 



KNOWLEDGE. 



June, 1913. 



of 225 H and D might be exposed at a distance of two feet 

 for two minutes to the light transmitted by it without 

 showing any fog on that account. Tests of this kind are 

 valuable in conjunction with spectroscopic ones from the 

 quantitative estimation of the time required to produce any 

 action. Of course, the time could be still further prolonged 

 by exposing further away, and so taking advantage of the 

 greater distance. But two minutes are ample for all practical 

 purposes, and by keeping the developing dish covered during 

 development no trouble could possibly occur through any 

 fault of the dark-room light. Unfortunately, orange paper 

 varies very much in colour as well as thickness, but that which 

 is fairly stout and of a dark tint should be chosen, and then 

 tested after the manner described. It will be noticed that the 

 cherry fabric is rather uneven in texture and also has a good 

 number of pin-holes in it, so that either two thicknesses would 

 have to be used or a combination made with canary medium, 

 when a fairly safe light would result for plates of average 

 rapidity. 



PHYSICS. 



By Alfred C. G. Egerton, B.Sc. 



THE HALL EFFECT. — Metals have the power of con- 

 ducting electricity and also heat ; their conductivity, however, 

 varies with the nature of the metal ; some metals, such as 

 tellurium, are feeble conductors, others, such as copper, offer 

 very low resistance indeed. The connection between con- 

 ductivity of electricity and that of heat is not at once evident, 

 though on the whole the thermal conductivity varies similarly 

 to the electrical with the nature of the metal. Electricity, 

 according to modern views, is conveyed along a conductor by 

 the electrons which pass from atom to atom. Heat is 

 conveyed as an increase in the kinetic energy of the molecules 

 of the substance ; but since the agination of the molecules 

 increases, so does the agitation of the particles contained 

 within them, viz., the electrons, and part of the thermal 

 energy conveyed along the conductor is due to the increased 

 kinetic energy of the electrons. This is especially the case for 

 the metals ; substances, such as sulphur, which are bad 

 conductors, conduct heat chiefly by the motion of the 

 molecules. It is possible to calculate approximately from 

 such considerations what the ratio of thermal conductivity to 

 electrical conductivity should be, and the ratio «r/<r at 10° to the 

 ratio KJa at 18° should be constant and have the value 1-28. 

 The results of experiments show that such a ratio is 

 approximately constant for many different metals — a 

 remarkable confirmation of the theory. 



Now if electricity is conveyed along a wire by the free 

 electrons which carry a negative charge, a magnetic field 

 should deflect such electrons across the breadth of the con- 

 ductor, and so one side will acquire a negative charge of 

 electricity. At the same time a diffusion of such charged 

 particles will take place from crowded parts to those less 

 crowded, and equilibrium will be attained when the effect due 

 to diffusion counterbalances that due to the deflection of the 

 electrons by the magnetic field, a difference of potential 

 between the upper and lower edges of the conductor being 

 the result. Such a difference of potential was observed by 

 Hall in 1879, and has since been studied by Nernst and 

 others. Experiments have been very difficult, as several 

 effects come into play besides the Hall effect. 



Kammerlingh Onnes, of Leyden, whose experiments on 

 liquefaction of gases are well known, has recently investigated 

 the Hall effect at very low temperatures. The effect obtained in 



the case of bismuth can be divided into two components : the 

 effect due to the magnetic field, which always gives a negative 

 charge, due to the deflection of the electrons ; the other effect 

 is a thermal effect inversely proportional to the absolute 

 temperature, which may reverse the sign of the charge due 

 to the magnetic field. Results of considerable importance to 

 the electron theory of metallic conduction may be expected 

 from such experiments. 



THE ROYAL SOCIETY'S CONVERSAZIONE.— On 

 May 7th the Royal Society held its conversazione, at which 

 many interesting exhibits were on view. Among such exhibits 

 may be mentioned the new lines which Professor Fowler has 

 found in the spectrum of hydrogen. Lines which have only 

 previously been seen from investigations of the spectrum of 

 the stars have been found when a very strong condenser 

 discharge is passed through a mixture of helium and hydrogen. 

 Another interesting exhibit was the micromanometer of Mr. 

 Fry, who arranges a stretched membrane so that its movement 

 twists a mirror suspended in a special manner : the pressure 

 differences being indicated by the deflection of a spot of light, 

 a difference of pressure of one-millionth of a millimetre of 

 mercury can be detected. 



ZOOLOGY. 



By Professor J. Arthur Thomson, M.A. 



SPOTTED NEGROES.— Q. I. Simpson and W. E. Castle 

 put on record an interesting peculiarity in human skin colour, 

 '" which made its appearance as a mutation or sport in a negro 

 family of the southern United States some sixty years ago, and 

 has shown itself fully hereditary through two generations of 

 offspring." The peculiarity is " piebaldness," the dark skin 

 being spotted with white in a fairly definite pattern. The 

 original ''mutant" was born in 1853, and her parents were 

 normally coloured negroes. She married a normal negro, and 

 had fifteen children, all of whom are living. The spottedness 

 behaves like a Mendelian character — as a simple dominant, 

 " the only peculiarity of the case being the excess of spotted 

 grandchildren over the expected one-half." Some of the 

 spotted descendants are now connected with " museums," 

 and the authors note that the piebaldness, being an economic 

 asset, is not likely to interfere with their racial increase. 



CLIMBING FISHES.— It is well-known that the tropical 

 fish, Periophthalmus, like its relative Boleophthalmus, 

 spends hour after hour out of water, squatting on the mud by 

 the sides of the tropical estuaries, or even climbing up on the 

 roots of the mangrove trees. But such climbing powers as 

 Periophthalmus possesses are far surpassed by a catfish 

 Arges marmoratus, which lives in the torrential rivers of the 

 Andes, where there is a rapid succession of falls, cascades, 

 and pot-holes. Under usual conditions A rges is a clumsy and 

 awkward swimmer, but for creeping and climbing in the 

 torrents it is wonderfully adapted. It anchors itself by its 

 suctorial mouth, and works itself upstream with the help of a 

 ventral bony plate bearing the ventral fins and equipped with 

 strong muscles which move it backwards and forwards. The 

 plate is studded with small sharp teeth pointing backwards. 

 The fishes climb up the smooth water-worn surfaces of deep 

 pot-holes, and have been known to ascend eighteen feet with- 

 out a slip or fall. 



NOTICES. 



ROYAL INSTITUTION.— The President has nominated 

 the following gentlemen as Vice-Presidents for the ensuing year: 

 — Dr. Henry E. Armstrong, the Right Hon. A.J. Balfour, J. H. 

 Balfour Browne, Esq., Sir William Crookes, Dr. Donald W. C. 

 Hood, the Right Hon. Sir James Stirling, Sir James Crichton- 

 Browne (Treasurer), and Alexander Siemens, Esq. (Secretary). 



SECONDHAND BOOKS ON NATURAL HISTORY. 

 — More than nine hundred and fifty books on Natural 

 History are included in Messrs. Henry Sotheran & Co.'s 

 recent catalogue, numbered 736. The prices appear to 

 be most reasonable and the volumes are on view at 

 43, Piccadilly. 



