Jan. 27, 1 88 1 ] 



NA TURE 



289 



With Prof. Ayrton I have done for water what Prof. James 

 Thomson did for carbonic acid ; we constructed in stiff paper a 

 surface or surfaces whicli rej- resent the relations of /, I'aiid t for 

 a given quantity of water-stuff. Three parts of the whole are 

 cylindric surfaces and divide space into three regions ; in one of 

 them the substance is in the form of ice, in another in the f rm 

 of water, in another in the ftrm of vapour; and they meet in 

 Thomson's triple point. Any one looking at this model must 

 feel that Prof. Ayrton was right in looking for the hot-ice slate 

 in a region bounded by imaginary productions of the all-tee, the 

 all-water, the all-vapour, and the above-mentioned three cylindric 

 surfaces beyond their lines of intersection. This is what Prof. 

 James Thomson did to indicate the state of water before 

 boiling by bumping begins. He assumed that the all-water 

 surface changed iirto the all-vapour surface gradually, and 

 not through a purely cylindric waler-zapour surface, and this 

 is really what Dr. Lodge himself dees for hot ice. That is, he 

 imagines the all-ice surface to chanf^e into the all-vapour surface 

 gradually, and not by sudden charges through a purely cylindric 

 ice-vapour surface. According to Mr. Ayrton the imaginary 

 production is even of a more complicated kind than Pr. Lodge 

 supposes, as the ice probably changes into unstable water before 

 it changes into steam. There can be m doubt that such imaginary 

 productions find their place in the fundamental equation of water, 

 but I cannot agree with Dr. Lodge in thinking that we have at 

 present an explanation of such unstable conditions. If his 

 explanation were satisfactory we ought to be able in the same 

 way to explain the unstable position wdiich prece>ies boiling by 

 bumping, and this w e cannot do. Where the explanation seems 

 to me to fail is in the assumption that the hot vnpour filling a 

 cavity, being of lower temperature than the >urface of the cavity, 

 is always at a pres>ure less than that of saturation, in spite of 

 the evaporation goiu'.' on. Now when w-e consider how large the 

 surface of a minute- cavity is as compared with its volume, the 

 very great increase in bulk when the solid is changed into vapour 

 and the lowering of temperature which the surface must undergo 

 on account of latent heat, we see that the condition which Dr. 

 Lodge assumes to be maintained during the whole experiment 

 w ould be instantaneously destroyed in a very minute cavity. In 

 explaining hot ice I am afraid that neither Prof. Ayrton nor Dr. 

 Lodge has given us more than Prof. James Thomson has given 

 in explaining '* boiling by bumping." The cause of the pheno- 

 menon is a molecular one probably, and must be left to the 

 guesses of molecular physicists. John Perry 



14, Talgarth Road, West Kensington 



Mr. Bottomley's Experiments with Vacuum Tubes and 

 the Aurora 

 Mr. Bottomley's extremely interesting experiments briefly 

 described in Nature, vol. xxiii. pp. 218 and 243, appear to 

 have a very important bearing on the question of atmospheric 

 electricity ; for if such high vacua are good conductors of elec- 

 tricity we have reason for thinling that the electrical conditions 

 of our globe w ill be very different from what we have been accus- 

 tomed to regard them. The layers of denser air surrounding the 

 conducting matter of the globe will act like the glass of Mr. Bot- 

 tomley's tubes in maintaining by a I.eyden-jar-like action any 

 difference of potential that there may be betvvetn their inner and 

 their outer surface. Again, in ihe piercing of the glass tube by a 

 nnnute spark, w- have the analogue of the lightning flash between 

 the clouds and the earth ; the insulating layer in each case giving 

 way, when, owing to an exce-sive increase in the surface density 

 of the charge at any point, the dielectric stress exceeds the 

 limits of the dielectric strength of the medium. The in- 

 ternal luminnus effects observed by Mr. Bottomley as the 

 result of change in the dis'ribution of the external charge 

 of electricity will be the physical analogues of the aurora, 

 with this difference, that they take place in the ultra-gaseous 

 interior, whereas in the case of our globe the luminous pheno- 

 mena take place in the ultra gaseous (/,£■. highly rarefied) exterior 

 regions of the atmosphere. It wfuld be interesting to learn 

 whether such discbarges ] -resent any other analogies with auroral 

 phev.omena. I should be particularly interested in learning 

 whether the conditions under which such luminous effects are 

 obtained give any support to the theory which I think to be the 

 only consi^tent one, that the aurora is due not to electrical dis- 

 charges from regions of less atmospheric density to regions of a 

 greater A&wiWy {qx vice versa), but to electrical discharges in a 

 region of pretty uniform (and small) density, and in which 



region differences of electric potential exist. According to this 

 view tlie auroral streaks which appear to be radial should in 

 reality lie approximately parallel to the earth's surface, and not 

 stand (as most persons imagine) normal to it. A series of hori- 

 zontal jjarallel lines drawn across the sky in a direction approxi- 

 mately north and south would necessarily appear to an observer 

 on the earth's urface foreshortened into a set of lines diverging 

 in fan-like forms at either the north point or the south point of 

 the horizon. Their divergence would therefore be apparent 

 only, like the ''beams" diverging from the sun at sunset on a 

 cloudy day, or Ike the beams of the rayons dii crepusculc, or 

 like the "1 adial streaks " which I have pointed out as frequently 

 accompanying rainbows. SiLVANUS P. Thomi'SON 



University College, Bristol, January 22 



P.S. — The behaviour of a hollow sealed glass tube containing 

 a conducting substance in its interior was noticed just one hun- 

 dred yeai s ago by Cavallo, who sealed up a glass tube in which 

 mercury was at its boiling-point, thus obtaining a fairly perfect 

 vacuum. — .S. P. T. 



The Geological Age of the Nortti Highlands of Scotland 



From the abstract of Proceediiisrs of the Geological Society 

 (January 5) I learn with surprise that Sir R. Murchison's inter- 

 pretation of the succession of the beds over the region north of 

 the Caledonian Canal is disputed, and that the relations of 

 the fo>silifen>us limestone of Durness to the quartzites "are" 

 (according to Pr. Callaway) "by no means satisfactorily esta- 

 blished, and that their conformity is rendered dubious by a 

 marked discordance of strike"; in fact that the limestone lies 

 in a synclinal basin amongst the quartzites, so that if the lime- 

 stone be nf Lower Silurian (" Arenig") age the quartzites and 

 schists must be older ; this I presume to be the inference Dr. 

 Callawav intends to draw, as he says there " is no proof of the 

 Lower Silurian age of the quartzite and newer series of flaggy 

 gneiss and schist " constituting the interior mountainous district. 



Havin'.; had an opiiortunity last sjsring of visiting the district 

 lying between Lochs ISroom and Inchard under the guidance of 

 Prof. Gcikie and in company with my colleague of the Irish 

 Survey, Mr. Symes, I take the opportunity offered by Pr. 

 Callaway's paper of expressing my entire concurrence in the 

 interpretation of the structure of the country given by my 

 late chief, wdiose elaborate and graphic descriptions in the 

 pages of the Quarterly Journal of the Geological Society (vols. 

 XV. and xvii.) will, I feel sure, never be invalidated. 



After seeing the clear infra-position cf the limestone to the 

 upper quartzite and schists first in the section at the Bridge of 

 Ault-Cf.rrv near Ullapool, then in the cliffs near Ullapool, next at 

 Inchnadamff and the head of Loch Assynt, then again in 

 the Forest of Arkle and the hills bordering Loch Stack, where 

 the limestone band is clearly interbedded letween the lower and 

 upper quartzites, and this latter as clearly y asses under the 

 schists of the interior, it required no further evidence to prove 

 that all the: e beds belong to one conforoiable formation; .and 

 that the geological age of the whole group is determined by the 

 fossilsdis^covei'ed by Mr. Charles Peach inthe limestone of Durness 

 or Assynt, and named by the late Mr. Salter. The geological 

 sequence is so clear throughout that region, and so entirely bears 

 out the description given by Murchison and Geikie, that "he 

 who runneth may read" ; and I have no hesitation in sayiiig 

 that the evidence that the Millstone Grit overlies the Carboni- 

 ferous Limestone, and that the New Red Marl overlies the New 

 Red S.Tndstone is not more clear than that the upper quartzites 

 and schi ts overlie the Assynt limestone. 



I wish to point out in conclusion that the trough-shaped 

 arrangement of the Lurne.ss limestone anl its faulted position, 

 described by Dr. Callaway, has already been described by 

 Murchison in he Quarterly journal, vol. xv. Any one visiting 

 the strand tract of country lying between Durness and Loch 

 Maree need have no better guide than the papers I have referred 

 to, and a good geological map. He will find that there is 

 lit'tle, if anything, to add to the details and conclusions there 

 given, and were'it not that. Dr. Callaway's objections seem to 

 "find support with some geologists of more e.xperience than 

 himself, it would not have been necessary to enter a caveat 

 against them. 



As re/ards the question whether in any part of the Highlands 

 of Scotland except along the western coast the Laurentian (or 

 "pre- Cambrian") rocks reappear, as has been stated or :ug- 

 gested, I do not wish to offer an opinion. As regards the region 



