on. in.] EVOLUTION AND DISSOLUTION. 3iy 



like conclusion. If its origin be purely igneous, tins rock may 

 have pre-existed as a liquid stream of matter surging beneath 

 the earth's solid envelope. If its origin be aqueous, its con- 

 stituent particles were once diffused over a wide area of 

 country, from which they were drawn together through sundry 

 rivulets and rivers, and here at last deposited as sediment. 

 In either case the process by which the rock has assumed 

 an individual existence has been a process of concentration. 

 And when it ceases to exist — whether it is blasted with 

 gunpowder, or chipped away with chisels, or eaten down 

 by runniug water, or ground to pieces by ocean waves, or 

 lowered through some long geologic epoch until it is melted 

 by volcanic heat — in any case its disappearance is effected 

 by a process of diffusion. 



But our account is as yet only half complete. In saying 

 that the career of any object, from its initial appearance 

 to its final disappearance, consists of a process of concentra- 

 tion followed by a process of diffusion, we omit an important 

 half of the truth. For in making such a statement, we are 

 attending only to the material elements of which objects 

 are composed; and we are leaving out of the account the 

 motions, both molar and molecular, which they exhibit, 

 and which constitute an equally important part of the entire 

 process. This defect we must now endeavour to remedy. 



A brief reconsideration of the examples already cited will 

 show us that universally the concentration of matter is ac- 

 companied by a dissipation of motion, while conversely the 

 diffusion of matter is attended by an absorption of motion. 

 The condensation of aqueous vapour into a cloud is effected 

 whenever it loses by radiation a greater quantity of that 

 kind of molecular motion known as heat than it is receiving 

 from the sun and the earth ; and when the loss of motion is 

 still more considerable, there occurs a further condensation 

 of the aqueous vapour into liquid rain. Conversely, when 

 solar radiance, direct or reflected, begins to impart to the 



