350 



NA TURE 



[Feb. 7, 1884 



At right angles is a tube of smaller diameter ; in this tube is a 

 stopper having a loop underneath from which is suspended by 

 two silk fibres, s, a piece of decarbonised iron, n n'. The 

 stopper when greased turns quite suioothly, and by rotating it 

 the needle cnn be easily placed in any direction with regard to 

 the tube. 



In the first place the tube was placed in the magnetic meridian, 

 and the needle of iron wire, « n, in the same direction ; tested 

 by means of a very small magnet, both ends were equally attracted 

 and not repelled, showing that the needle had been thoroughly 

 decarbonised ; this was done by heating it to redness for many 

 hours in peroxide of iron, prepared by burning its oxalate. 



The dischaige w.ns in the first instance passed from the ring to 

 the point, so that the needle was in the dark space ; no magnetism 



h4@> 



CCCE 



was developed in the needle, which would have been the case if 

 the discharge had had a spiral motion as we have often observed 

 and de-cribed to be sometimes the case. It was indeed with the 

 object of ascertaining this fact that the apparatus had been made. 



The needle was now placed at right angles to the tube, and 

 the point made positive ; after a few trials at different exhausts 

 a beautiful tongue-shaped stratification was obtained, and it was 

 then possible by altering the amiamt of the current to make the 

 apex of a stratum imfiiiige on one or the other end of the needle. 

 Fig-. 2 and 3 ; on whichever end the stratum touched, that end 

 was pushed away by it, shoeing clearly that the balance of 

 forces which hold together the molecules composing a stratum 

 are sufticient to render it viscous, and unyielding to a small 

 resistance. 



Geological Society, January 23. — R. Etheridge, F.R.S., 

 vice-i resident, in the chair. — George Henry Nelson and John 

 rhilip Spencer were elected FelloAvs of the Society. — The fol- 

 lowing communications were read : — On the Serpentine and 

 associated rocks of Porthalla Cove, by J. H. Collins, F.G.S. — 

 Outline of the geology of Arabia, by C. M. Doughty. Com- 

 municated by Prof. T. G. Bonney, F.R.S. The author 

 described the general outline of the geology of a consider- 

 able district of the western part of Arabia, over which he had 

 travelled. It was not in his power to enter into details, espe- 

 cially as regarded the sedimentary rocks, because the circum- 

 stances under which his journey was undertaken made it im- 

 possible to bring back si-eciinens. There was, however, con- 

 siderable simplicity in the geological structure of the country. 

 The igneous rocks consisted of granites and basalts, the latter 



breaking through the former. The sedimentary rocks, which 

 are newer than the granites and, in fact, rest upon them, consist 

 of — [a) A yellowish sandstone, with stains of a reddish or greenish 

 colour and veins (-f ironstone. In this, for example, the rock- 

 tombs, &c. , of Pctra have been excavated. These substances, 

 in the author's opinion, may be traced as far as Medina, and 

 occur all about Kasim. They often weather in a singular way ; 

 pebbles are scarce in them ; fossils he had ni;t seen, (b) The 

 limestone contains bands of flint, and appears to be identical 

 with that which occurs in Palestine, and is, he thinks, probably 

 of Cretaceous age. (c) Of much later date is a coarse flat gravel 

 which overspreads a considerable tract of country, as, for 

 example, at Mount Seir in Edom, altogether about 250 square 

 miles. The flints are doubtless derived from the limestone, and 

 are often polished by drifting sand. It occurs on plateaux at 

 very considerable elevations above the sea, sometimes forming 

 the highest ground in the neighbourhood ; and sections had 

 shown this gravel to be more thin 20 feet deep. In it the 

 author had discovered two or three flint weapons of palaeolithic 

 type, rude, but very like those of Hoxne or St. Acheul. The 

 granite by its aspect and mode of occurrence recalls that of 

 .Sinai. It is cut by dykes of basalt ; and now and then the 

 author had observed other intrusive igneous rocks, which he 

 must be content to classify as traps. The dykes of basalt, how 

 ev?r, were not the only modes of occurrence of this rock ; there 

 were considerable flows of basaltic lavas and occasional small 

 craters. These volcanic districts bear the name of Harra ; the 

 principal are the Aneyrid, the Khaybar, and the Kesshub. The 

 last lies between Nejd Arabia and the Mecca country. These 

 masses of lava, &c., are comparatively modern ; eruption, 

 indeed, has in one or two localities occurred in historic times, 

 and steam has been seen to issue from certain craters. 



Physical Society, January 26. — Prof. Clifton in the chair. 

 — New member, Yung Free, Secretary of the Chinese Legation. 

 — Prof. Clifton announced that Lady Siemens had presented a 

 portion of the late Sir William Siemens's library to the Society. — 

 The meeting, which was at first a special meeting to consider the 

 resolution that it is expedient for the past presidents of the 

 Society to be permanent vice-presidents, having agreed to this 

 resolution, was constituted an ordinary meeting, and Professors 

 Ayrton and Perry described and exhibiied their new ammeters 

 and voltmeters, also a non-sparking key. The well-known 

 ammeters and voltmeters of the authors used for electric light 

 work are now constructed so as to dispense with a constant, and 

 give the readings in amperes and volts \\'ithout calculation. 

 This is effected by constructing the instruments so that there is a 

 falling off in the controlling magnetic field, and a considerable 

 increase in the deflecting magnetic field. The deflections are 

 thus made proportional to the current or E.M.F. measured. 

 The ingenious device of a core or soft iron pole piece adjustable 

 between the poles of the horseshoe magnet is used for this 

 purpose. By means of an ammeter and voltmeter used con- 

 jointly, the resistance of part of a circuit, say a lamp or heated 

 wire, can be got by Ohm's law. Professors Ayrton and Perry's 

 non-sparking key is designed to prevent sparking with large 

 currents. It acts by introducing a series of resistance-coils 

 determined experimentally one after the other in circuit, thereby 

 cutting off the spark. — Dr. C. R. Alder Wright, F.R.S. , read a 

 paper on the electromotive force set up during interdiffusion, 

 being the result of experiments m.ade by himself and Mr. ( . 

 Thompson to determine the effect of varying densities of solu- 

 tions used in voltaic cells on their E.M.F. 's. The observations 

 were made by constructing the cells of pure materials and 

 opposing them so that the differential E.M.F.'s could be mea- 

 sured by galvanometer or quadrant electrometer, when solutions 

 of different densities were employed. The following general con- 

 clusions were reached : (l) In any two fluid cells containing solu- 

 tions of two metallic salts and plates of the respective metals 

 contained therein, an increase of strength in the solution sur- 

 rounding the plate acquiring the higher potential in virtue of the 

 normal action of the cell causes an increment in the potential 

 difference between the two plates ; and the opposite effect is 

 I produced by an increment in the strength of the solution surround- 

 ing the other plate. (2) A law of summation holds, expressible 

 thus : the effect of the sum of a series of changes in the strengths 

 of the solutions in a two-fluid cell is equal 10 the algebraic sum 

 of the effects of each change severally. The author considered 

 this law very fully; and pointed out that "diffusion cells "act at 

 I least partly after the fashion of thermo-couples transforming into 

 ] electric energy a certain amount of sensible heat. 



