HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



the earthy and alkaline metals, and that its prevailing high 

 temperature was due to chemical processes. Davy's explana- 

 tion afterwards found favour with De la Rive and Charles Lyell. 

 Volger explained the heat of the earth partially as a product of 

 the pressure which the higher mountain-systems exert upon 

 the regions underlying them, partially as a result of the 

 chemical changes constantly going on in the earth's crust ; 

 and the Ultraneptunist chemist, Mohr, in his Gcschichte <frr 

 Erde (1866), explained the internal heat of the earth as a 

 transmutation of the sun's energy by chemico-physical pro- 

 cesses. 



Lichtenberg and Franklin thought that the firm earth's crust 

 surrounded a half-gaseous, half-viscous mass of very great 

 density. This opinion was accepted by Herbert Spencer, and 

 lias since been placed upon the basis of the Mechanical 

 Heat Theory by Ritter (1879) and the geographer Zopprit/ 

 (1882). According to this theory, there is under the firm 

 crust a zone of viscous material, then a zone of more fluid 

 material ; the earth's nucleus itself, however, is said to consist 

 of an outer gaseous part, in which the gases are in their normal 

 condition, and an inner gaseous part, in which they are above 

 the critical point. Owing to the excessive pressure, the gaseous 

 material of the earth's nucleus is said to become no less dense 

 than liquid or solid bodies. 



The English physicist, Hopkins, has been one of the most 

 famous champions of the theory of the earth's rigidity. Seeing 

 that the earth behaves as a firm mass in response to the 

 attraction of other bodies in the universe, and that the 

 phenomena of precession and nutation are not consistent 

 with an even partially fluid or plastic condition of the earth, 

 Hopkins concluded that the earth has been rendered for the 

 most part solid, in consequence of the cooling and of the great 

 pressure within the earth. Like Hopkins, Poisson and Ampere 

 (1868) were also of opinion that the earth's nucleus could not 

 be fluid, as otherwise the attraction of the moon would cause 

 gigantic tidal waves to take place in the firm crust. 



The physicists, Lord Kelvin (Sir W. Thomson) and George 

 Darwin, also attribute great importance to the enormous 

 pressure existing in the interior of the earth, and the con- 

 solidation of the nucleus from this cause. Darwin agrees 

 with Hopkins in respect of the behaviour of the earth relative 

 to the sun and the moon, and tries to prove by calculation that 



