io8 



PHYSIOGRAPHY. 



[chap. 



however, be effected by the aid of some of the common 

 metals of every-day life. Iron, for instance, answers the 

 purpose well enough, provided the metal be sufficiently 

 heated to stimulate its attraction for oxygen until it can 

 overcome the bonds which tie the oxygen and hydrogen 

 together. Fig. 28 represents a common method of effecting 

 the decomposition of water by means of iron, a is an iron 

 tube, such as a gun-barrel, strongly heated in a furnace b ; 

 water is boiled in the vessel c, and its vapour conducted 

 through the iron tube ; in traversing the heated iron the 



Fig. 28. — Decomposition of water by means of heated iron. 



Steam is broken up, its oxygen combining with the iron to 

 form an oxide, whilst its hydrogen is set free and may either 

 be burnt directly, or collected in d. This experiment shows 

 that steam, or water-gas, has the same chemical composi- 

 tion as liquid water. A gaseous compound of oxygen and 

 hydrogen goes in at a, and free hydrogen comes out at d ; 

 the oxygen having been fixed by the iron which forms an 

 oxide, not indeed the same as that which exists in iron-rust, 

 but an oxide identical with that which forms the natural 

 loadstone, whence it is called magnetic oxide of iro7i. 



In these experiments the hydrogen only has been set free ; 



