Professor De la Rive on Electricity. S73 



chciTiical action which the unoxidizcd surface of the nucleus 

 must exert upon the water, and other agents which may come 

 into contact with it. This hypothesis, founded on the nature of 

 earthy substances, which are only metallic oxides, seems also to 

 give a satisfactory explanation of all the other phenomena of 

 the pliysical earth. It has nevertheless been attacked, and ano- 

 ther has been proposed to be substituted, which consists in the 

 supposition, that the globe has been originally, in consequence 

 of the high temperature, in a state of complete fluidity ; that it 

 has then cooled down by degrees ; that this cooling has pro- 

 duced the solidification of the exterior crust, which, always going 

 on, produces an increase in the thickness of the shell, which 

 notwithstanding is very feeble in comparison to the extent of 

 the central liquid mass. This theory derives support princi- 

 pally from the spheroid form of the earth, which could only re- 

 sult as the effect of rotation on a liquid mass ; also fj:om the 

 regular increase of temperature which is constantly observed so 

 soon as we penetrate a little below the surface of the soil ; from 

 the appearance of volcanoes, the products of which arise from the 

 burning materials shut up in the interior of the globe ; from 

 earthquakes, which are the effects of efforts made by the ma- 

 terials, upborne by the elastic force of gases and vapours, to es- 

 cape ; — these are a few of the phenomena, amongst many others, 

 which are invoked as arguments in favour of the opinion to 

 which we have been just adverting. 



To explain the terrestrial currents, recourse must be had, in 

 this theory, to the calorific action of the solar rays : it may be 

 supposed that the surface of the earth is covered with thermo- 

 electrical currents ; which are produced by the differences of 

 temperature, which exist between those parts of the surface 

 which have been heated by the sun, and those which have not 

 yet been heated. But this difference of temperature is so feeble ; 

 the substances which form the surface of the earth are such bad 

 conductors of heat and electricity, and the heat itself is so irre- 

 gularly distributed, that it is difficult to find, in this cause, a 

 source of the currents sufficiently intense, and disposed in a 

 manner sufficiently regular, to explain the phenomena of terres- 

 trial magnetism. Probably we may, with more likelihood, attri- 

 bute to these thcrmo-electrical currents, developed by the ca- 

 lorific action of the sun upon the surface of the earth, the diur. 



