18 CONSTITUENT MATERIALS OF THE EARTH 



matter implies a change of some kind with regard to the con- 

 dition of the heat. Had this power continued to act with its 

 full original repulsive energy, the process of agglomeration 

 by attraction could not have gone on. We do not know 

 enough of the laws of heat to enable us to surmise how the 

 necessary change in this respect was brought about ; but we 

 can trace some of the steps and consequences of the process. 

 Uranus would be formed at the time when the heat of our 

 system's matter was at the greatest, Saturn at the next, and so 

 on. Now this tallies with the exceeding diifuseness of the 

 matter of those elder planets, Saturn being not more dense or 

 heavy than the substance cork. It may be that a sufficiency 

 of heat still remains in those planets to make up for their 

 distance from the sun, and the consequent smallness of the 

 heat which they derive from his rays. And it may equally 

 be, since Mercury is nearly thrice the density of the earth, 

 that its matter exists under a degree of cold for which that 

 planet's large enjoyment of the sun's rays is no more than a 

 compensation. Thus there may be upon the whole a nearly 

 equal experience of heat amongst all these children of the 

 sun. Where, meanwhile, is the heat once diffused through 

 the system, over and above what remains in the planets ? 

 May we not rationally presume it to have gone to constitute 

 that luminous envelope of the sun, in which his warmth- 

 giving power is now held to reside ? It may have simply 

 been reserved to constitute, at the last, a means of sustaining 

 the many operations of which the planets were destined to be 

 the theatre. 



The tendency of the preceding considerations is to impress 

 the notion that our globe is a specimen of all the similarly- 

 placed bodies of space, as respects its constituent matter and 

 the physical and chemical laws governing it, with only this 

 qualification, that there are possibly shades of variation with 

 respect to the component materials, and undoubtedly with 

 respect to the conditions under which the laws operate, and 

 consequently the effects which they produce. Thus, there 

 may be substances here which are not in some of the other 

 bodies, and substances here solid may be elsewhere liquid or 



