Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/239

Rh CONDITION or E_r.rH’s 1xrEn1or..] resistance to the passage of heat. while the more dense and crystalline offer the least resistance. The resistance of opaque white quartz is expressed by the number 114, that of basalt by 273, while that of cannel coal stands very much higher at 1538, or more than thirteen times that of quartz.‘ It is evident also that, from the texture and structure of most rocks, the conductivity must vary in different directions through the same mass, heat being more easily conducted along than across the “grain,” the bedding, and the other numerous divisional surfaces. Experiments have been made to determine these variations in a number of rocks. Thus, the conductivity in a direction transverse to the divisional planes being taken as unity, the conductivity parallel with these planes was found in a variety of magnesian schist to be 4028. In certain slates and schistose rocks from central France the ratio varied from 1: 2'56 to 1 : 3952. Hence in such ﬁssile rocks as slate and mica-schist heat may travel four times more easily along the lines of cleavage or foliation than across them.3 In reasoning upon the discrepancies in the rate of increase of subterranean temperatures, we must also bear in mind that certain kinds of rock are more liable than others to be charged with water, and that, in almost every boring or shaft, one or more horizons of such water-bearing rocks are met with. The effect of this interstitial water is to diminish thermal resistance. Dry red brick has its resist- ance lowered fron1 680 to 405 by being thoroughly soaked in water, its conductivity being thus increased (38 per cent. A piece of sandstone has its conductivity heightened to the extent of 8 per cent. by being wetted.3 Mr Mallet has contended that the variations in the amount of increase in subterranean temperature are too great to permit us to believe them to be due merely to differences in the transmission of the general internal heat, and that they point to local accessions of heat arising from transformation of the mechanical work of compression, which is due to the constant cooling and contraction of the globe.4 But it may be replied that these variations are not greater than, from the known divergences in the conduc- tivities of rocks, they might fairly be expected to be. Probable C'omlz'tz'on of the Earth’s Interior.-—Vario11s theories (mostly fanciful) have been propounded on this subject. There are only three which merit serious con- sidcration. One of these supposes the planet to consist of a solid crust and a molten interior. (2.) The second holds that, with the exception of local vesicular spaces, the globe is solid and rigid to the centre. The third con- tends that, while the mass of the globe is solid, there lies a liquid substratum beneath the crust. 1. The arguments in favour of internal liquidity may be summed up as follows. The ascertained rise of tem- perature inwards from the surface is such that, at a very moderate depth, the ordinary melting point of even the most refractory substances would be reached. At 20 miles the temperature, if it increases progressively, as it does in the depths accessible to observation, must be about 1760° F ahr. 5 at 50 miles it must be 4600°, or far higher than the fusing- point even of so stubborn a metal as platinum, which melts at 3080" Fahr. All over the world volcanoes exist from which steam and torrents of molten lava are from time to time erupted. Abundant as are the active volcanic vents, they form but a small proportion of the whole which have been in operation since early geological time. It has been inferred therefore that these numerous funnels of 1 Herschel and Lebonr, Brit. Assoc. Rep, 1875, p. 59. 2 Jannettaz, Bull. Soc. Géol. de France (April—June, 1874), tom. ii. p. 264 ; “ Report of Committee on Thermal Condnctivities of Rock,” L’-rit. Assoc. Rcp., 1875, p. 61. 3 Herschel and Lebonr, Brit. Assoc. Rep., 1875, p. 58. ‘ “ Volcanic Energy,” Phil. Trans., 1875. GEOLOGY 225 communication with the heated interior could not have existed and poured forth such a vast amount of molten rock, unless they drew their supplies from an immense internal molten nucleus. (c.) When the products of volcanic action from different and widely—separated regions are compared and analysed, they are found to exhibit a remarkable uni- formity of character. Lavas from Vesuvius, from Hecla, from the Andes, from Japan, and from New Zealand present such an agreement in essential particulars as, it is contended, can only be accounted for on the supposition that they have all emanated from one vast common source? (¢l.) The abundant earthquake shocks which affect large areas of the globe are maintained to be inexplicable unless on the supposition of the existence of a thin and somewhat ﬂexible crust. These arguments, it will be observed, are only of the nature of inferences drawn from observations of the present constitution of the globe. They are based on geological data, and have been frequently urged by geologists as supporting the only view of the nature of the earth’s interior compatible with geological evidence. 2. The arguments against the internal ﬂuidityof the earth are based on physical and astronomical considerations of the greatest importance. They may be arranged as follows :- (a.) Argument from precession and nutation.—The pro- blem of the internal condition of the globe was attacked as far back as the year 1839 by the late Mr Hopkins of Cambridge, who endeavoured to calculate how far the planetary motions of precession and nutation would be inﬂuenced by the solidity or liquidity of the earth’s interior. He found that the precessional and nutational movements could not possibly be as they are if the planet consisted of a central ocean of molten rock surrounded with a crust of 20 or 30 miles in thickness, that the least possible thick- ness of crust consistent with the existing movements was from 800 to 1000 miles, and that the whole might even be solid to the centre, with the exception of comparatively small vesicular spaces ﬁlled with melted rock.5 M. Delaunay, in a paper on The Hypothesis of the Interior Fluidity of the G'lobe,7 threw doubt on Hopkins’s views, and suggested that, if the interior were a mass of sufficient viscosity, it might behave as if it were a solid, and thus the phenomenon of precession and nutation might not be affected. Sir William Thomson, who had already arrived at the conclusion that the interior of the globe must be solid, and acquiesced generally in Hopkins’s conclusions, pointed out that M. Delaunay had not worked out the problem mathematically, otherwise he could not have failed to see that the hypothesis of a viscous and quasi-rigid interior “breaks down when tested by a simple calculation of the amount of tangential force required to give to any globular portion of the interior mass the precessional and nutational motions which, with other physical astronomers, he attributes to the earth as a whole.”3 Sir William, in making this calculation, holds that it demonstrates the earth’s crust down to depths of hundreds of kilometres to be capable of resisting such a tangential stress (amounting to nearly Tluth of a gramme weight per square centimetre) as would with great rapidity draw out of shape any plastic ‘substance which could properly be termed a viscous ﬂuid. “ An angular distortion of 8" is produced in a cube of glass by a distorting stress of about ten grammes weight per square centimetre. Ve may therefore safely conclude that the rigidity of the earth’s interior substance could not be less than a millionth of the rigidity of glass without very sensibly augmenting the lunar nineteen-yearly nutation.”° 5 See D. Forbes, “On the Nature of the Interior of the Earth," Popular Science Review, April 1869. 5 Phil. Tra72s., 1839; Researches -in Physical Geology, 18394842; Brit. Assoc. R€])., 1847. 7 Comptes Rendus, July 13, 1868. 3 Nature, February 1, 1872. 9 1300- 01"-n P- 258- X. — 29