HYDROGEN. 21 



next most important element in nature. It is de- 

 rived from two Greek words, signifying ^' water''' 

 and " to /orm," because with oxygen it forms water ; 

 and, when exhibited, it is generally obtained from 

 the decomposition of water. It is without taste or 

 odour, and is the most inflammable of all substances. 

 When mixed with atmospheric air, it burns with a 

 violent explosion. Hydrogen is 14 times hghter 

 than atmospheric air, and for this reason it is used 

 to inflate balloons. It is obtained for this purpose 

 by mixing together sulphuric acid, iron filings, and 

 water, which results in the decomposition of the 

 latter, and the consequent liberation of the hydro- 

 gen. 



In order to form a correct idea of the quantity of 

 hydrogen on the globe, we must bear in mind that 

 water consists of two volumes of hydrogen and one 

 of oxygen, and, consequently, so far as water is 

 concerned, the quantity of hydrogen is double that 

 of oxygen. With respect to the waters of the 

 ocean, we must deduct about four per cent, for the 

 amount of salts in solution ; but with this allow- 

 ance, when we reflect that the ocean has an aver- 

 age depth of three miles, and covers three fifths of 

 the surface of the globe, we perceive that there must 

 be a vast amount of hydrogen locked up. To this 

 must be added the immense quantity of water con- 

 tained in rocks* and in the interior of the earth, 

 which, though chiefly derived from the atmosphere 

 in the form of rain, and filtering through immense 



* The following is a statement of the average per centum of 

 water, in all the simple minerals containing it, that usually 



enter into the composition of rocks : 



Sulphate of lime .... 19.88 



Serpentine 12.75 



Diallage 8.20 



Talc and stealite . . . 4.20 



Pyroxene . . • . . 3.74 



Mica 2.65 



Quartz 1.62 



