Refrigeratmi of the Earth, 217 



empty space are not to be procured *; but the radiation of heat, 

 at low temperatures, is so slight that it is scarcely sensible at 

 100° of Fahrenheit's thermometer, a temperature twice as great 

 as the medium temperature of the surface of the globe at this 

 time. The temperature of the earth's surface has been shewn 

 by Fourier to bo as constant as are the dimensions of its orbit, 

 and the period of its annual revolution. Laplace observes, that 

 our planet has undergone no contraction of size during the last 

 2000 years ; consequently there has been no sensible refrigera^ 

 tion during that period, and the last seculum of M. de Beau- 

 mont has already extended to more than twice the length of a 

 millennium. 



Another argument, or rather postulate, has been adduced in 

 favour of central heat, — the fusion of unstratified rocks, and 

 their forcible injection into the stratified. 



Gentlemen, I have confessed to you again and again, that T 

 am not aware, nor has any one as yet informed me, by what test 

 stratified and unstratified rocks can be distinguished ; the only 

 test I know is the good will and pleasure of those who make the 

 distinction. The followers of Pluto seize and appropriate to 

 his use as many rocks as they think proper. By virtue of such 

 seizure, these rocks become necessarily unstratified : why so ? 

 because if stratified they would be no longer Plutonic. Strati- 

 fication I know is a question to be determined not by the senses 

 but by the fancy ; otherwise, I would say, that the magnificent 

 range of basaltic cliff, which extends from the county of Derry 

 along the coast of Antrim as far as Fairhead, is as distinctly 

 stratified as any mountain-limestone, oolite or chalk, in Great 

 Britain. 



- However, I waive this objection, as it leads me away from 

 my subject, and return to the consideration of central heat. 

 Have those who believe in this agent ever taken into their ac- 

 count the nature of the substances said to have been fused ? 

 Many of the trap rocks, not all of them, (for the family is large, 

 and many of its members have been introduced into it, not by 

 nature but by adoption), I attribute to the agency of the causes 

 which have produced lava, causes which, comparatively speak- 



• See Comparative View of the Huttonian and Neptunian Systems of 

 Geology. 



