A HISTORY OF SCIENCE 



would necessitate such a regularity in every part of 

 the ring, and in its cooling, that this phenomenon is 

 extremely rare; and the solar system affords us, in- 

 deed, but one example namely, in the ring of Saturn. 

 In nearly every case the ring of vapor was broken into 

 several masses, each moving at similar velocities, and 

 continuing to rotate at the same distance around the 

 sun. These masses would take a spheroid form with a 

 rotatory movement in the direction of the revolution, 

 because their inner molecules had less velocity than 

 the outer. Thus were formed so many planets in a con- 

 dition of vapor. But if one of them were powerful 

 enough to reunite successively by its attraction all the 

 others around its centre of gravity, the ring of vapor 

 would be thus transformed into a single spheroidical 

 mass of vapor revolving around the sun with a rota- 

 tion in the direction of its revolution. The latter case 

 has been that which is the most common, but never- 

 theless the solar system affords us an instance of the 

 first case in the four small planets which move be- 

 tween Jupiter and Mars ; at least, if we do not sup- 

 pose, as does M. Olbers, that they originally formed 

 a single planet which a mighty explosion broke up 

 into several portions each moving at different veloc- 

 ities. 



"According to our hypothesis, the comets are stran- 

 gers to our planetary system. In considering them, 

 as we have done, as minute nebulosities, wandering 

 from solar system to solar system, and formed by 

 the condensation of the nebulous matter everywhere 

 existent in profusion in the universe, we see that when 

 they come into that part of the heavens where the sun 



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