286 



NATURE 



{July 21, 1887 



On one side of a screen conducted to earth, were the two main 

 electrodes, from which the current of the large battery passed. 

 On the other side were two auxiliary electrodes connected to the 

 two poles of a small battery. Whenever the main current 

 passed, the small battery was found to send a steady current 

 which could be measured. The smallest electromotive force 

 which was observed to send a current under these conditions was 

 one-sixth of a Leclanche. 



An electromotive force of one-sixth of a Leclanche is about one- 

 quarter of a volt, and a current has thus been obtained in a gas 

 from an electromotive force which could not maintain a current 

 through water. 



An electromotive force of O'l volt gave doubtful results, but 

 this was probably due to the experimental difficulty of detecting 

 the current. 



In some previous experiments, which, however, were not quite 

 free from objection on other grounds, the lowest electromotive 

 force for which the currents could be measured was 0'2 volt. 



The experimental arrangement which is the best for the quali- 

 tative investigation of the effect is not the best for quantitative 

 measurements, and I have therefore not endeavoured to follow 

 out to any great extent the quantitative laws of these currents 

 produced by low electromotive forces. I may give, however, 

 some facts which I have observed. The intensity of the current 

 depends on a great many circumstances. 



(i) It increases rapidly with the'intensity of the main discharge, 

 and also with a reduction of pressure, as far as I have tried it 

 (that is about half a millimetre). 



(2) The intensity of the current from the auxiliary battery 

 increases less rapidly than the electromotive force. 



(3) In some experiments, in which one of the electrodes of the 

 auxiliary battery was a copper wire and the other a copper 

 cylinder, the current was nearly always considerably stronger 

 when the larger surface was the kathode. 



(4) Anything that facilitates the diffusion of gas from the main 

 current to the auxiliary electrodes will increase the strength of 

 the current observed. In some experiments, in which the 

 screen separating the two fields was made of wire gauze instead 

 of tinfoil, the currents were stronger than those given above. 



These experiments show conclusively that there is nothing 

 peculiar in the gaseous state of a body to prevent any electro- 

 motive force, however small, to produce a current, if a finite 

 electromotive force is required under ordinary circumstances, the 

 fact cannot be accounted for, as Edlund and others have done, 

 by a special surface resistance which has to be overcome by 

 a finite difference of potential at the surface. 



I think the facts are very well accounted for by the theory 

 which I have proposed in my last paper. If the two atoms of a 

 gas making up the molecule are charged with opposite electri- 

 cities, but are held together in addition by molecular forces, a 

 finite force is required to overcome the latter. But as soon as 

 that force is overcome and the atoms themselves are set free to 

 diffuse and constitute a current, these atoms will be able to 

 follow any electromotive force which we may apply. If, then, 

 we have auxiliary electrodes, these electrodes will establish their 

 electric field, which we can never screen off completely from 

 any other part of the vessel except by closed surfaces. The 

 atoms, with their positive and negative charges, will diffuse 

 across to the auxiliary electrodes and give off their electricity to 

 them. No finite difference of potential is required in the 

 auxiliary electrodes, because, even if there is work done in 

 making an atom interchange its positive for negative electricity, 

 that work is undone again at the other pole, where atoms of a 

 similar kind interchange negative for positive electricity. 



I should like, in conclusion, to point out an important appli- 

 cation of these results. I have last year obtained by calculation 

 results which seem to show that the principal cause of the diurnal 

 variation of terrestrial magnetism is to be looked for in the upper 

 regions of the atmosphere. Prof. Balfour Stewart at various 

 times suggested that the air-currents in these regions may, owing 

 to the lines of force of terrestrial magnetism, have electric 

 currents circulating in them. 



The difficulty against this supposition always seemed to me to 

 lie in the fact that the electromotive forces required to start a 

 current were larger than those which could possibly exist in the 

 atmosphere. But as there are very likely continuous electric 

 disturbances going on, such as we observe in aurorse and thunder- 

 storms, the regions within which these discharges take place 

 would act as conductors for any additional electromotive force 

 however small, so that any regular motion, such as tidal 



motions, could very well produce periodic effects affecting our 

 magnetic needles. 



If these original discharges increase in importance, then, 

 according to the results obtained in this paper, the currents due 

 to the smaller periodic causes would increase also, and they may 

 increase in a very rapid ratio. We know that the electric dis- 

 charges in the upper regions of the atmosphere are considerably 

 stronger at times of many sunspots, and this may account for the 

 fact that at those times the amplitude of the daily oscillation of 

 the magnetic needle is considerably increased. 



I have had considerable assistance in these experiments from 

 my assistant, Mr. Stanton, to whom my best thanks are due. 



Geological Society, June 23.— Prof. J. W. Judd, F.R.S., 

 President, in the chair. — The following communications were 

 read: — On nepheline rocks in Brazil, with special reference to 

 the association of phonolite and foyaite, by Mr. Orville A. 

 Derby. The author refers to the phonolites and associated 

 basalts of Fernando Noronha, a deep-sea island off the north- 

 eastern shoulder of the continent of South America. Nepheline 

 rocks of a somewhat different character are abundantly developed 

 on the mainland, and under conditions favourable for throwing 

 light on the relations existing between the granitic type, foyaite, 

 and the other members of the group. There are some moun- 

 tains near Rio de Janeiro composed of these rocks, as is also the 

 peak of Itatiaia, 3000 metres high, the loftiest mountain of 

 eastern South America. A cursory examination of some of 

 these localities having shown an apparent relation between 

 foyaite, phonolite, trachyte, and certain types of basalt, Mr. 

 Derby determined to visit the Caldas region, where a railway 

 under construction gave unusual facilities for examining this 

 series. A fine development of foyaite, phonolite, and tuff was 

 found, associated with several types that have not yet been met 

 with in the other localities. The existence of a leucite basalt 

 was recognized. The bulk of the paper was devoted to a 

 detailed description of these railway-sections, and the following 

 deductions are drawn : — (i) The substantial identity, as regards 

 mode of occurrence and geological age, of the Caldas phonolites 

 and foyaites. (2) The connexion of the latter through the 

 phonolites with a typical volcanic series containing both deep- 

 seated and aerial types of deposits. (3) The equal, if not greater, 

 antiquity of the leucite rocks as compared with the nepheline 

 rocks, whether felsitic, as phonolite, or granitic, as foyaite. (4) 

 The probable Palaeozoic age of the whole eruptive series. The 

 President said it was seldom that a paper containing such im- 

 portant facts was presented to the Society. It was reserved to 

 Mr. Derby to have proved that plutonic rocks containing 

 nepheline (foyaite) passed into volcanic masses which were true 

 phonolites. This Mr. Derby had clearly established by ob- 

 servations in the field. He had also shown that leucite existed 

 in rocks of Palssozoic age, thus rendering untenable the last 

 stronghold of those who insisted on making geological age a 

 primary factor in petrographical classification. He alluded also to 

 the value of the independent determinations of Prof Rosenbusch. 

 Mr. Bauerman said he had been over portions of the ground 

 with the author, and was glad to add his testimony to the value 

 of the paper. He spoke of the importance, in a geological 

 sense, of these generalizations. It was remarkable how highly 

 crystalline masses of rock pass over into a sort of phonolite. 

 These were associated with Palaeozoic masses, which were pre- 

 Permian, or at least pre-Triassic. He alluded to the difficulty 

 of investigating Fernando Noronha, and also to the difficulties 

 attendant upon the investigation of rocks in Brazil, which were 

 subject to such an enormous amount of local alteration. Prof. 

 Bonney also expressed his sense of the value of the paper. He 

 alluded to the comparative rarity of nepheline and leucite rocks, 

 and to the confusion in the nomenclature. He was reminded of 

 the nepheline rocks near Montreal, where dolerite was broken 

 through by nepheline syenite, associated with tephrites and 

 phonolites. Although there might be a doubt here, these rocks 

 were most probably of Silurian age ; but the evidence in Brazil 

 was still clearer as to the Palaeozoic age, and he believed that, in 

 the case of some other masses, the evidence had satisfied the 

 Canadian geologists. He alluded also to the nepheline rocks in 

 the Katzen-Buckel, where there was a similar passage^ from 

 coarse-grained to fine-grained. Dr. Hatch said that in this case 

 leucite was clearly shown to be of Palaeozoic age, and regarded 

 the paper as a step towards the better classification of this group 

 of rocks. Prof. Seeley asked for evidence as to the identifica- 

 tion of the leucite. The President thought there was no possi- 

 bility of a mistake in this respect. As regards the rocks of the 



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