64 HABIT AJSTD INTELLIGENCE. [cHAP. 



and that every aggregate — that is to say, every star and 

 planet — has been produced by the aggregation, or conden- 

 sation, of matter that was previously in a diffused form, 

 perhaps somewhat like that of water in a cloud. 



It is to be observed that aggregation may be repeated 

 an indefinite number of times. The original nebula may 

 have condensed into masses no larger than meteors, and 

 these may have coalesced into planets ; the planets may 

 ultimately swell the mass of the sun ; suns may rush 

 together ; and so on, absolutely without limit, if the 

 universe is infinite. 



The smallest excess of density in one part of a nebula 

 above the rest will produce at that place a centre of aggre- 

 gation; and round it the nebulous matter will condense 

 into a star or a planet, much in the same way that a cloud 

 condenses into raindrops. 

 Trausfor- All the energy in the nebula was at first potential 

 raer°v in energy. When motion began in consequence of the mutual 

 the process gravitation of its parts, part of this was changed into 

 sation. energy of motion ; and when any of the moving parts 

 struck against each other and coalesced into masses, de- 

 priving each other of part of their momentum, a part of 

 their energy of motion was transformed into heat. Such 

 was the origin of the internal heat of the earth (and in all 

 probability of all planets, except those which are so small 

 as to rank as mere meteors) ; ^ and such is the process by 

 which, according to the metoric theory, solar heat is stiU 

 originated. 



'C^ 



Solar and We thus come to the wonderful but highly probable 

 heat have conclusion, that solar heat and volcanic heat are of the 



the same game origin.^ 

 origin. 



Potential ^^ ^^^ ^®®^ calculated by Professor Helmholtz, that all 

 energy of the potential energy due to the mutual gravitation of the 



the origi- 

 nal nebula. 1 It is well known that the moon's surface contains marks of past 



volcanic activity, and this appears to be not j'et extinct. (Quarterly 



Jounial of Science for July 1867, p. 383.) 



^ The chemical theory of volcanic action is in my opinion completely 



disproved by the comparative absence of hydrogen flames in volcanic 



eruptions ; for that theory requires the action of water as an oxidising 



agent, and this involves the liberation of hydi'ogen. 



