AND THEORY OF THE HEAVENS. 93 



been in a vacuum, and at the same time to diminish 

 infinitely the resistance which they could make to each 

 other. But these particles could of themselves assume 

 such a state of attenuation, as cannot be doubted if we 

 have any knowledge of the expansion which matter goes 

 through when it is transformed into vapour, or keeping 

 to the heavens when we consider the attenuation of 

 matter in the tails of comets which, with such an enor- 

 mous thickness across as exceeds the diameter of the 

 earth by at least a hundred times, are yet so transparent 

 that the small stars may be seen through them, whereas 

 our atmosphere when it is illuminated by the sun does 

 not allow this at a height which is many thousand times 

 less, 



I conclude this chapter by adding an analogy which 

 by itself alone can raise the present theory of the 

 mechanical formation of the heavenly bodies above the 

 probability of an hypothesis, and carry it up to a formal 

 certainty. If the sun is composed of particles of the 

 same primary matter out of which the planets have been 

 formed, and if the only difference between them consists 

 in this, that in the sun matter of all kinds is accumulated 

 without distinction, whereas in the planets it has been 

 distributed at different distances according to the con- 

 dition of the density of its kinds : then, if we consider 

 the matter of all the planets as combined together, from 

 their total mixture there would result a density almost 

 equal to the density of the body of the sun. Now this 

 necessary consequence of our system finds a happy con- 

 firmation which M. de Buffon, that deservedly celebrated 

 philosopher, has instituted between the densities of the 

 whole matter in the planets and the sun. He found a 



