CHEMISTRY OF THE ROCKS. 



63 



vapors of nearly all of them correspond strictly to their combin- 

 ing numbers. The following table therefore will show the 

 relative positions in the atmospheric strata, of some of the most 

 important elements, with the weights of their atoms in hydrogen 

 units, their vapor-densities compared with air, and the solid spe- 

 cific gravities of some of them as compared with water : 



The atomic weight of any substance, simple or compound, 

 multiplied into the specific gravity of hydrogen (.069) will give 

 the specific gravity of that substance in gaseous form as in 

 the above table, and which ought very nearly to agree with that 

 found by actually weighing the gas when that is practicable. 



It will be noticed from this table that the elements were ar- 

 ranged in positions most suitable for their combination and de- 

 position, both in geological order and in the probable order of 

 their condensation from vapors. Oxygen and silicon, which 

 doubtless composed more than four-fifths of the entire bulk of 

 the gases, were separated from each other only by the elements 

 that were needed to make up the silicates. Their compound, 

 silica, is involatile, and even infusible by itself, under any de- 

 gree of heat that we can command. The same is true of lime 

 and the earlier-formed silicates. Therefore it is impossible to 

 decide from their volatility which of these substances would 



