102 The Interior of the Earth. [Sess. 



By these considerations we are led to the conclusion that if 

 this radiation into space has gone on for untold ages, there 

 must have been a time when the earth was in a molten state 

 throughout. 



Assuming this to be the case, the condition of the earth's 

 interior at the present time must depend on the manner in 

 which it has cooled, and Mr Hopkins has shown that the 

 earth's mass must now be in one of three states : these are — 



1. A solid crust with a fluid interior. 



2. A solid crust and a solid nucleus with a fluid inter- 



stratum. 



3. A solid throughout, with or without fluid spaces or 



cavities 

 Hopkins concluded that the balance of probability was in 

 favour of No. 3. He bases his argument on astronomical 

 reasoning. Sir W. Thomson agrees with Mr Hopkins's con- 

 clusion that the earth is either solid or possesses so thick a 

 crust that it amounts to the same thing. He thinks that the 

 tide -producing power of the sun and moon is so great that 

 unless the thickness of the crust exceeds 2000 miles it would 

 yield to the strain. Sir W. Thomson finally concludes that 

 the mass of the earth " is on the whole more rigid than a 

 continuous globe of solid glass of the same diameter." 



Professor G. H. Darwin investigated this problem twice, — 

 first in 1879, when he concluded that the rigidity of the 

 earth was not so great as Thomson supposed, but at the same 

 time he stated "that no very considerable portion of the 

 interior can even distantly approach the fluid condition." In 

 his second investigation of this extremely complex subject, he 

 expresses his result as generally confirmatory of Thomson's 

 view as to the effective rigidity of the earth's whole mass. 



Mr Fisher, in his ' Physics of the Earth's Crust,' suggests 

 the existence of a fluid substratum which does not consist en- 

 tirely of melted rock, but of a mixture of melted rock with 

 dissolved gases, such as water, vapour, hydrogen, &c. This 

 supposition is said to meet the requirements of astronomers, 

 and has a certain amount of evidence to support it. Large 

 volumes of steam and hydrogen are frequently observed to be 

 evolved from active volcanoes, which may be considered to have 

 some connection with the fluid substratum. 



