86 ON THE STKUCTUJKE OF ATOMS. 



The nebular theory of LaPlace does this so completely that 

 every physicist feels that it is a true one, whether it can be abso- 

 lutely proved or not. 



If it is true, and the matter of our solar system was ever thus 

 expanded to fill the planetary spaces, then this matter must have 

 been very nearly homogeneous ; that is, all its ultimate particles, 

 however small, mast have been of nearly the same weight or 

 attractive force. For if it had consisted of atoms of much dif- 

 ferent weights, like those of our sixty -five elements, many of 

 which are over two hundred times heavier than the lightest, then 

 the densest must inevitably have collected at the center of the 

 mass. There would have been a very great increase in the den- 

 sity of the planets according to their nearness to the sun. The 

 outer ones would have been composed of hydrogen ; the inner 

 ones perhaps of the minerals lime, silica, &c., and the sun would 

 probably have retained all the heavier metals, as silver, mercury, 

 lead, gold, and platinum. It is almost inconceivable that the 

 earth should have really had such a mixture of the lightest and 

 densest of the elements, ranging in comparative weights from 1 

 up to 230, or that the sun should be surrounded by such im- 

 mense volumes of hydrogen, the lightest of all the elements, 

 while underlying it should be the vapors of sodium, magnesium, 

 calcium, and iron, belonging to the category of lighter elements. 



The supposition which most fully meets the requirements of 

 the case we have been presenting, is that at least some of the 

 lighter and simpler elements of each planet and satellite contin- 

 ued in some manner to be formed and brought out during the 

 cooling and condensation of each body. It is a very bold specu- 

 lation, but after all it is sometimes necessary to form what are 

 called "working hypotheses," and then work up to them, and 

 ascertain what there may be of truth in them. 



It is now an almost every day occurrence with the astronomers 

 to watch through the spectro-telescope the eruptions of hydrogen 

 gas on the surface of the sun. Enormous volumes of this gas 

 are frequently seen to burst up through the chromosphere and 

 ascend with amazing velocity to a height sometimes of 200,000 

 miles. These outbursts often carry up with them the glowing 



