100 Prof. Heiincssy o?i the Influence of the 



the vitreous condition up to that of complete crystalline struc- 

 ture, it is at once apparent that the influence of changes of the 

 state of molecular aggregation of such masses must exercise an 

 important influence on the general structure of the earth, as well 

 as the slow cubical contraction of the entire mass during the 

 process of its refrigeration. 



In my second memoir, I showed the way in which the former 

 species of contraction would affect the internal structure of the 

 earth supposed to consist of a solid shell and included nucleus 

 of fluid. The process of solidification of the fluid, commencing 

 from the centre and ending at the surface, according to the 

 views of Poisson, was proved to be impossible ; and the only way 

 in which the process could take place, was shown to be by suc- 

 cessive additions of matter from the surface of the nucleus to the 

 inner surface of the solid shell. Each outer stratum of the 

 nucleus would thus successively become a stratum of the shell. 

 But the density of any stratum of equal pressure in the fluid 

 depends on the pressure of all the strata by which it is enve- 

 loped, and therefore the removal of these strata in regular suc- 

 cession, gradually tending to decrease the pressure, must decrease 

 the density of the stratum in question. This action, operating 

 on all the strata of the nucleus, will manifestly give it a tendency 

 to enlarge its volume so as to fill up the space left by the con- 

 traction of its exterior strata. This expansion of the nucleus 

 will evidently be accompanied by a diminution of its mean den- 

 sity, and has been shown to be also attended with a change in 

 the law of density in going from its centre to its surface. The 

 density will vary less rapidly as the solidification of the mass 

 advances, and the fluid will tend to become more homogeneous. 

 .The mass of the shell is at the same time continually augmented 

 in a corresponding degree by the successive additions it thus 

 receives at its inner surface, so that the aggregate effect of the 

 process of solidification is the removal of matter from the centre 

 towards the surface of the earth. 



An admirable example of the effect of internal forces on the 

 solidification of a cooling mass of fused igneous rock, which will 

 serve in some measure to illustrate my remarks, has been de- 

 scribed by Mr. Darwin in his * Naturalists' Voyage*/ it presents 

 a case where the forces tending to expand the liquid enclosed in 

 the first solidified envelope are extremely energetic, and the order 

 of the phsenomena is thus very clearly exhibited. He noticed in 

 several places in the Island of Ascension volcanic bombs, which 

 have been shot through the air while in a fluid state, and which, 

 spinning around their centres as they passed through their course, 

 have usually assumed a rounded shape. . He then remarks, that 



* Page 493. 



