26 



Third. They likewise argue, that the peculiar appearance of 

 lavas all over the world indicates that they proceed from a com- 

 mon source, These indications are merely that of a rock altered 

 by a solvent, and that solvent, be it igneous or aqueous, is go- 

 verned by the laws of chemical action, and therefore no argu- 

 ment in support of central heat. 



Finally, they contend that on no other hypothesis can we 

 account for the change of climate indicated by the fossils. In 

 another chapter we shall endeavour to show an ample cause for 

 the latter. 



Not only is the above doctrine of central heat unsupported by 

 the arguments brought forward in its favour, but the conse- 

 quence of such an igneous nucleus according to the laws which 

 regulate the circulation of heat through fluid bodies, would be, 

 that the crust of the earth, instead of increasing in thickness, 

 would be altogether melted, even were it two hundred miles, 

 much less sixty. (See Plate XX.) A cold crust and an incan- 

 descent nucleus are incompatible, and contrary to the known 

 laws of terrestrial physics ; yet it has been attempted to show 

 by experiment and mathematical calculations that these are ne- 

 cessary truths in a body circumstanced as the earth really is. 



It is without doubt one of the most dangerous errors in every 

 science to attempt to allow mathematical refinement to usurp 

 the place of careful experiment ; and it would not be difficult to 

 point out many instances of the injury which physical inquiry 

 has sustained from the too-prevalent reliance upon the supposed 

 power of mathematical investigations to alleviate the toil of in- 

 ductive research ; thus too often substituting the laws of geo- 

 metry for those of physics, and leading the mind completely 

 astray from the legitimate effects of the latter. 



To make the hypothesis of an incandescent nucleus appear 

 somewhat consistent, the following experiment is suggested : — 

 " If one end of a bar of metal, a few feet long, be plunged in the 

 fire, while the other end is wrapped in a wet cloth, the one end 

 may be ignited to any desired degree, while the other can be kept 

 at any required temperature above a certain point, depending on 

 the heating and cooling powers applied to the ends of the bar, 

 its length, and the conducting and radiating powers of the metal. 

 Instead of the metal bar, submit to the same heat a bar of 

 stone or a rod of glass ; in these cases, unless the bar be very 





