160 PROFESSOR W. THOMSON ON THE SECULAR COOLING OF THE EARTH- 



9. The chemical hypothesis to account for underground heat might be re- 

 garded as not improbable, if it was only in isolated localities that the tempera- 

 ture was found to increase with the depth ; and, indeed, it can scarcely be doubted 

 that chemical action exercises an appreciable influence (possibly negative, how- 

 ever) on the action of volcanoes ; but that there is slow uniform " combus- 

 tion," " eremacausis," or chemical combination of any kind going on, at some 

 great unknown depth under the surface everywhere, and creeping inwards 

 gradually as the chemical affinities in layer after layer are successively saturated, 

 seems extremely improbable, although it cannot be pronounced to be absolutely 

 impossible, or contrary to all analogies in nature. The less hypothetical view, 

 however, that the earth is merely a warm chemically inert body cooling, is clearly 

 to be preferred in the present state of science. 



10. Poisson's celebrated hypothesis, that the present underground heat is due 

 to a passage, at some former period, of the solar system through hotter stellar 

 regions, cannot provide the circumstances required for a pakTeontology continuous 

 through that epoch of external heat. For from a mean of values of the conduc- 

 tivity, in terms of the thermal capacity of unit volume, of the earth's crust, in three 

 different localities near Edinburgh, which I have deduced from the observations on 

 underground temperature instituted by Principal Forbes there, I find that if the 

 supposed transit through a hotter region of space took place between 1250 and 5000 

 years ago, the temperature of that supposed region must have been from 25° to 

 50° Fahr. above the present mean temperature of the earth's surface, to account 

 for the present general rate of under-ground increase of temperature, taken as 1° 

 Fahr. in 50 feet downwards. Human history negatives this supposition. Again, geo- 

 logists and astronomers will, I presume, admit that the earth cannot, 20,000 years 

 ago, have been in a region of space 100' Fahr. warmer than its present surface. But 

 if the transition from a hot region to a cool region supposed by Poissox took place 

 more than 20,000 years ago, the excess of temperature must have been more than 

 100° Fahr., and must therefore have destroyed animal and vegetable life. Hence, 

 the farther back and the hotter we can suppose Poisson's hot region, the better 

 for the geologists who require the longest periods ; but the best for their view is 

 Leibnitz's theory, which simply supposes the earth to have been at one time an 

 incandescent liquid, without explaining how it got into that state. If we suppose 

 the temperature of melting rock to be about 10,000° Fahr. (an extremely high 

 estimate), the consolidation may have taken place 200,000,000 years ago. Or, 

 if we suppose the temperature of melting rock to be 7000° Fahr. (which is more 

 nearly what it is generally assumed to be), we may suppose the consolidation to 

 have taken place 98,000,000 years ago. 



11. These estimates are founded on the Fourier solution demonstrated below. 

 The greatest variation we have to make on them, to take into account the differ- 

 ences in the ratios of conductivities to specific heats of the three Edinburgh rocks. 



