104 The Interior of the Earth. [Sess. 



yield to compression at such great depths and under tremend- 

 ous pressure ; but from our present knowledge of the behaviour 

 of different substances when subjected to great squeezing force, 

 we may pretty safely assume that beyond a certain rather 

 small shrinkage, matter will not further yield to additional 

 pressure. Putting aside, then, the possibility of unlimited 

 compressibility, we must infer that the interior portions of the 

 globe are composed of materials of a different hind to those 

 appearing at the surface. 



Careful study of the substances brought up through volcanic 

 vents shows that even at moderate depths there exist materials 

 differing widely in density and chemical composition. The 

 lightest lavas have a density of 2*3, and the heaviest of over 

 3. We have also the clearest proof that materials of even 

 greater density are brought to the surface by volcanic agency. 



Here, again, the study of astronomy and the results of 

 spectrum analysis in recent years afford us valuable help. 

 We know that the earth is one of a family of globes revolving 

 round the sun as a centre, and that they have a close relation- 

 ship with one another. As far as spectrum analysis has been 

 applied, we have every reason to believe that not only the 

 members of the solar system, but the more distant bodies of 

 the universe, are all composed of the very same elementary 

 substances as those which enter into the composition of our 

 earth. 



Until within quite recent times the space occupied by the 

 members of the solar system was generally considered to be a 

 great void. We now know that the interplanetary space 

 simply swarms with small bodies, all travelling in definite 

 paths under fixed laws, and acting and reacting on one another, 

 sometimes coming into collision. Now and then some of 

 these small bodies, meteorites as they are usually called, find 

 their way to the earth, and from a study of their composition 

 we derive the best evidence we have of the composition of the 

 planetary bodies outside our earth. 



No chemical element has been found in any meteorite 

 yet examined which is not also found on the earth, and out of 

 the 70 odd elements known to science I believe 22 have been 

 detected at various times in different specimens of meteorites. 



Though the number of elementary substances is so large, 



