558 GEOLOGY. 



of condensation is large. However this may be, it seems safe to infer 

 that in so far as the volume of the interior mass is dependent on heat 

 resistance, the loss of existing heat leads to the generation of new heat. The 

 amount of this new heat must be enough, together with the residual 

 heat and the other forces of resistance, to match the new condensational 

 forces. The molecular and sub-molecular forces of resistance other 

 than heat, are probably responsible for some large part of the resistance 

 to the increased condensational force, but how much is not determined. 



All resistance perhaps due to motion. — As now interpreted, the force 

 of resistance of heat is due to the impact of the flying particles of the 

 heated matter. The other forms of resistance to compression have not 

 usually been interpreted in this way, but the tendency of recent investi- 

 gation is to place them in the same dynamic class. A cold solid body 

 offers resistance to compression that is in no obvious way dependent 

 on heat motion. In small bodies this resistance is immeasurably greater 

 than the self-gravity of the body. It is so great that it can only be par- 

 tially overcome by any force which human ingenuity can bring to bear 

 upon it. This form of resistance has thus, not unnaturally, come to be 

 regarded as approximately immeasurable, and perhaps as grading into 

 actual immeasurability, and as resting back upon the actual contact 

 of irreducible atoms. But the recent researches which have developed 

 grounds for the conception that even the atoms are composite, lead to 

 the further conception that their resistance to compression is dependent 

 on the movement of their constituent corpuscles or electrons. This en- 

 courages the broad conception that the whole of tlie resistance to com- 

 pression arises from molecular, atomic, and sub-atomic motions, of 

 which heat is merely one form. 



While all this is yet on the frontier of physical progress, these con- 

 ceptions may well be recognized in framing interpretations of the agen- 

 cies which determine the volume of the earth, and which control the 

 changes that take place in it from age to age. The result of their com- 

 bined action at any stage is a state of temporary eqiiilihriiim between 

 gravity, aided by the molecular, atomic, and sub-atomic attractions, 

 on the one hand, and heat, aided by the molecular, atomic, and sub- 

 atomic resistances, on the other. The vital jDroblem is to ascertain 

 the original condition of balance between these antagonistic forces, and 

 the changes which have affected that balance since. The original state 

 of balance is necessarily a matter of hypothesis, and the best thaV 



