May 1 6, 1895] 



NA TURE 



67 



Among the remaining papers read were: — "Science as a 

 Subject in Girls" Schools," by Miss F. E. Hunt; "The 

 Curriculum of Secondary Education," by D. H. HoUidge ; 

 ' The Technical Element in a State System of Education," by 

 Antony St. Ledger ; " A Contribution towards the Study of the 

 Relation of I';thics and Science," by the Rev. J. S. Pollock ; 

 "The Importance of Mental Science as a Guide in Primary 

 Education," by James Rule. 



The business of the .Association concluded with a meeting 

 of the General Council, at which the following recommendations, 

 among others, were adopted : — 



( 1 ) That the committee for the investigation of the thermo- 

 dynamics of the voltaic cell be reappointed without grant. 



(2) That the report of the Seismological Committee be printed, 

 and that the committee be reappointed and allowed a grant of 

 £,\o towards the cost of the erection of the instruments presented 

 by Dr. \o\\ Rebeur-l'aschwitz at Tiniaru. 



(3) That the following be a committee — namely, Messrs. F. M. 

 Bailey, R. L. Jack, A. Gibb Maitland, \. Meston, C. W. De 

 \'is, and H. Tryon — to investigate the geology, land flora, and 

 natural resources generally of the islands and islets of the Great 

 Barrier Reef. 



(4) That the New Zealand Government be asked to set apart 

 -Stephen's Island, Cook Strait, as a reserve for the Tuatara 

 Lizard. 



(5) That the committee for the investigation of glacial deposits 

 in -Australasia be Messrs. Hutton, R. L. Jack, R. Tate, R. M. 

 John.ston, F. W. E. David (secretary), G. Sweet, J. Shirley, 

 W. Houchins, El. G. Hogg, E. J. Dunn, A. Montgomery, and 

 E. F. Pittman. 



(6) That a committee — consisting of Messrs. H. C. Stanley, 

 A. li. Brady, Thomas Parker, Prof. Warren, Prof. Kernot, 

 Henry Moncriefi', and James Fincham — be appointed to inquire 

 into the habits of the teredo, and the best means of preserving 

 timber or structures subject to the action of tidal waters. 



(7) That the connnittee on psycho-physical research be ap- 

 pointed without a grant. 



The next meeting of the .Association will be held at Sydney 

 in 1S97, underlhe presidency of Prof. Liversidge, and the follow- 

 ing meeting will take place at Melbtjurne. 



ELECTRIFICATION OF AIRLAND THERMAL 

 CONDUCTIVITY OF ROCK AT DIFFERENT 

 TEMPERATURES.^ 



(I.) "OX THE ELECTRIFICATION OF AIR. ' 



§ I. /CONTINUOUS observation of natural atmospheric 

 ^-^ electricity has given ample proof that cloudless air 

 at moderate heights above the earth's surface, in all weathers, is 

 electrified with very far from homogeneous distribution of electric 

 density. Observing, at many times from May till -September, 

 1859, with my portable electrometer on a flat open .sea-beach of 

 Brodick Bay in the Island of -Vrran, in ordinary fair weather at 

 all hours of the day, I found the difference of potentials, 

 between the earth and an insulated burning match at a height of 

 9 feet above it (2 feet from the miinsulated metal case of the 

 instrument, held over the head of the observer), to vary 

 from 200 to 400 Daniell's elements, or as we may now 

 say volts, and often during light breezes from the east and 

 north-east, it went up to 3cxx) or 4000 volts. In that place, 

 and in fair weather, I never founil the potential other than 

 positive (never negative, never even down to zero), if for brevity 

 we call the earth's potential at the place zero. In perfectly clear 

 weather under a sky sometimes cloudless, more generally some- 

 what clouded, I often observed the potential at the 9 feet height 

 to vary from about 300 volts gradually to three or four times 

 that amount, and gradually back again to nearly the same lower 

 value in the course of about two niinules.t I inferred that these 

 gradual variations must have been produced by electrified masses 

 of air moving past the place of observation. I did not remark 

 then, but I now see, that the electricity in these moving masses 

 of air must, in all probability have been chiefly ]X)sitive to cause 

 the variations which I observed, as I shall explain to you a little 

 later. 



* Two communic.itioiis to the: IMiiloj-uphicil Society of Glxsgow meeting, 

 in the N.-itur.tl Philosophy I.ecrure-ruoni of the University of Glasgow, 

 March 27, " On the Elcctritic-ition of Air" : "On the Thermal Conductivity 

 IRock at different temper.itures." 



t " Klectrost.itics arul Magnetism "VS i iWilllani Thomson), xvl. §§ aSi. 

 J82. 



NO. 1333, VOL 52] 



§ 2. .Soon after that time a recording atmospheric electrometer * 

 which I devised, to show by a ])holographic curve the continuous 

 variation of electric potential at a fixed point, wa.s established at 

 the Kew Meteorological Observatory, and has been kept in 

 regular action from the commencement of the year 1861 till the 

 present time. It showed inces.sant variations quite of the same 

 character, though not often as large, as those which I had 

 observed on the sea-beach of -Arran. 



Through the kindness of the -Astronomer Royal, I am able to 

 place before you this evening the photographic curves for the 

 year 1893, produced by a similar recording electrometer which 

 has been in action for many years at the Royal Observatory, 

 Greenwich. They show, as you see, not infrequently, during 

 several hours of the day or night, negative potential and rajiid 

 transitions from large positive to large negative. Those were 

 certainly times of broken weather, with at least showers of rain, 

 or snow , or hail. But throughout a very large proportion of the 

 whole time the curve quite answers to the description of what I 

 observed on the -Arran sea-beach thirty-six years ago, except that 

 the variations which it shows are not often of so large amount 

 in proportion to the mean or to the minimums. 



§ 3. Thinking over the subject now, we see that the gradual 

 variations, minute after minute through so wide a range as the 3 

 or 4 to I, which I frequently obser\'ed, and not infrequently 

 rising to twenty times the ordinary minimum, must have been due to 

 positively electrified masses of air, within a few hundred feet ot 

 the place of observation, wafted along with the gentle winds of 

 5 or 10 or 15 feet per second which were blowing at the time. 

 If any comparably large quantities of negatively electrified air 

 had been similarly carried past, it i.s quite certain that the 

 minimum observed potential, instead ot being in every case 

 jiositive, would have been frequently large negative. 



§ 4. Tw o fundamental questions in respect to the atmospheric 

 electricity of fair weather force themselves on our attention : — 

 (i) What is the cause of the prevalent po.sitive potential in the 

 air near the earth, the earth's potential being called zero } (2) 

 How comes the lower air to be electrified to different electric 

 densities whether po.sitive ornegative in difl'erent parts? Observa- 

 tions and laborator}' experiments made within the last six c^r 

 eight )ears, and particularly two remarkable discoveries made by 

 Lenard, which I am going to describe to you, have contributed 

 largely to answering the second of these questions. 



§ 5. In an article " On the Electrification of -Air by a Water- 

 jet," by Magnus Maclean and Makita Goto,t e.xperiments were 

 described showing air to be negatively electrified by a jet of 

 water shot vertically down through it from a fine nozzle into a 

 basin of water about 60 centimetres below it. It seemed natural 

 to suppose that the observed electrification was produced by the 

 rush of the fine drops through the air ; but Lenard conclusively 

 proved, by elaborate and searching experiments, that il was in 

 reality due chiefly, if not wholly, to the violent commotions of 

 the drops impinging on the water surfiice of the receiving basin, 

 and he found that the negative electrification of the air wa.s 

 greater when thev were allowed to fall on a hard slab of any 

 material thoroughly wetted by water, than when they fell on a 

 yielding surface of water several centimetres deep. He had been 

 engaged in studying the great negative potential which had been 

 found in air in the neighbourhood of waterfalls, and which had 

 generally been attributed to the inductive action of the ordinary 

 fine weather electric force, giving negative electricity to each 

 dro]^ of water-spray before it breaks away from conducting com- 

 munication with the earth. Before he knew Maclean and (loto's 

 jjaper, he had ftmnd strong reason for believing that that theory 

 wa.s not correct, and that the true explanation of the electrifica- 

 tion of the air must be found in .some physical action not hitherto 

 discovered. A less thorough inquirer might have been satisfied 

 with the simple explanation of the electricity of waterfalls 

 naturally suggested by -Maclean and Goto's result, and might 

 have rested in the lielief that it was due to an electrifying effect 

 produced by the rush of the broken water through the air ; but 

 Lenard made an indejiendent ex|>erimental investigation in the 

 Physical Laboratories of Heidelberg and Bonn, by which he 

 learned that the seat of the negative electrification of the air 

 electrified is the lacerated water at the foot of the fall, or at any 

 rocks against which the water impinges, and not the multitudinous 

 interfaces between air and water falling freely in drops 

 through it. 



§ 6. It still seems worthy of searching inquiry to find 



• " Electrostatics and Magnetism '" xvi. §§ 271,292. 

 t l^hilosofthical Magazine, iSoo. second half.year. 



