210 ANNUAL OF SCIENTIFIC DISCOVERY. 



that of a screen of glass ; which keeps up the temperature beneath it, 

 directly, by preventing the escape of radiant heat, and indirectly by 

 hindering the condensation of the aqueous vapor in the air confined 

 beneath. 



Now, we have only to bear in mind that there are the best of reasons 

 for believing that, during the earlier geological periods, all of the carbon 

 since deposited in the forms of limestone and of mineral coal existed 

 in the atmosphere in the state of carbonic acid, and we see at once an 

 agency which must have added greatly to produce the elevated tem- 

 perature that prevailed at the earth's surface in former geological peri- 

 ods. Without doubt, the great extent of sea, and the absence or rar- 

 ity of high mountains, contributed much to the mild climate of the Car- 

 boniferous age, for example, when a vegetation as luxuriant as that 

 now found in the tropics flourished within the frigid zones ; but to 

 these causes must be added the influence of the whole of the carbon 

 which was afterwards condensed in the forms of coal and carbonate of 

 lime, and which then existed in the condition of a transparent and per- 

 manent -gas, mingled with the atmosphere, surrounding the earth, and 

 protecting it like a dome of glass ; To this effect of carbonic acid it is 

 possible that other gases may have contributed. The ozone, which is 

 mingled with the oxygen set free from growing plants, and the marsh 

 gas, which is now evolved from decomposing vegetation under condi- 

 tions similar to those then presented by the coal-fields, may, by their 

 great absorptive power, have very well aided to maintain at the earth's 

 surface that high temperature the cause of which has been one of the 

 enigmas of geology. 



SECULAR COOLING OF THE EARTH. 



Professor William Thomson, in a communication to the Koyal So- 

 ciety of Edinburgh, says, " The fact that the temperature of the 

 earth increases with the depth below the surface implies a continual 

 loss of heat from the interior by conduction outwards, through, or into 

 the upper crust. Since the upper crust does not become hotter from 

 year to year, there must, therefore, be a secular loss of heat from the 

 whole earth. It is possible that no cooling may result from this loss of 

 heat, but only exhaustion of potential energy, which, in this case, could 

 scarcely be other than chemical affinity between substances forming 

 part of the earth's mass. But it is certain that either the earth is be- 

 coming, on the whole, cooler from age to age, or that the heat conduct- 

 ed out is generated in the interior by temporary dynamical action 

 (such as chemical combination). To suppose, as Lyell has done, that 

 the substances combining together, according to the chemical hypothe- 

 sis of terrestrial heat, may be again separated electrolytically by thermo- 

 electric currents, due to the heat generated by their combination, and 

 thus the chemical action and its heat continued in an endless cycle, 

 violates the first principles of natural philosophy, in exactly the same 

 manner and to the same degree as to believe that a clock constructed 

 with a self-winding movement may fulfil the expectations of its ingen- 

 ious inventor by going forever. 



"Adopting as the more probable, the simpler hypothesis that the earth 

 is merely a heated body cooling, and not, on the whole, influenced to 

 any sensible degree by interior chemical action, the author applies 



