M. Haidinger on the Original Formation of Aerolites. 455 



there would, at all events, remain an interior space of 403 miles, 

 within which the formation of another such spherical shell might 

 possibly proceed. Nevertheless it is not to be expected that 

 further condensation out of the primitive molecular state should 

 go on without some disturbance in a medium of such a tempera- 

 ture as prevails in planetary space. If contraction produces an 

 actual internal vacuum, a violent disruption of the crust falls 

 within the bounds of possibility. Admitting that a compensa- 

 tion of temperature by conduction or communication of heat to 

 have already taken place, and supposing every solid shell to be 

 hermetically sealed under a high temperature, an event quite op- 

 posite to the above-mentioned one might be expected with some 

 degree of probability. Gases developed within this shell and 

 brought to high tension, might indeed cause a violent explosion, 

 exactly like that arising from ignited gunpowder enclosed within 

 a hollow projectile. 



What is the actual cause of the densities of the planetary 

 bodies within our solar system being so different from each 

 other ? Does it merely arise from the natural correlation of the 

 elements composing them, as in our globe, or from a progressive 

 development in the earlier stages of their existence ? The den- 

 sities of these bodies are expressed by the following numbers : — 

 Mercury, 671 ; Earth, 5 -44; Mars, 5*15; Venus, 5'02; the 

 Moon, 3-37; Sun, 1-37; Jupiter, 1 -29 ; Neptune, 1-2; Uranus, 

 0-98; Saturn, 1-75. 



Olbers is known to have first enounced the hypothesis that the 

 minor planets Ceres and Pallas, discovered by Piazzi and himself, 

 were probably mere fragments of a pre-existing and larger planet. 

 After the discovery of Juno and Vesta, Lagrange* investigated 

 the intensity of an explosive force sufficient to rend a planet into 

 pieces, in order to permit a fragment of it to become a comet, 

 or, to use a more accurate expression, move in an orbit similar 

 to a comet. He found that an impulsion equal to the velocity of 

 a cannon-ball multiplied by 12-15, that is 16,800-21,000 

 feet per second (the velocity of a cannon-ball being 1400 feet a 

 second, and equal to that of a point at the equator in its 

 diurnal rotation), would be sufficient to throw the fragments of a 

 planet (the radius of its orbit being supposed to be equal to the 

 distance of our globe from the Sun multiplied by 100) into pro- 

 gressive or retrogressive, elliptical or parabolical comet-orbits — 

 the greater number of them even into hyperbolical ones, so that, 

 after their first perihelion, they would disappear for ever from our 

 system f. 



* " Sur l'Origine des Cometes." Lu au Bureau des Longitudes, le 29 

 Janvier, 1812. — Connaissance des Terns, fyc. pour l'an 1814, Avril 1812, 

 p. 211. 



t Baron Reichenbaeh has expressed an opinion that meteorites may 



