108 IRON. 



carbonate of lime. This is the most useful of all the 

 kinds of steel, and employed for razors, surgeons*" 

 instruments, and similar purposes ; its grain is the 

 most compact, and it takes the highest polish. 



It is the particular property of steel to become 

 extremely hard, if it be heated red hot, and then 

 suddenly plunged into cold water ; but different 

 instruments made of steel require to be of different 

 degrees of hardness ; and they are, therefore, sub- 

 jected to a process called tempering ^ which is heat- 

 ing them again to a certain point after having been 

 hardened. The tempering of steel, for some pur- 

 poses, is a delicate process. 



A polished bit of steel, when heated with access 

 of air, acquires very beautiful colours. It first be- 

 comes of a pale yellow, then of a deeper yellow, 

 next reddish, then deep blue, and at last bright 

 blue. At this period it becomes red hot, and the 

 colours disappear: at the same time the metallic 

 scales, or the black imperfect oxide of iron which is 

 formed, incrusts its surface. All these different 

 shades of colour indicate the different tempers the 

 steel has acquired by the increase of heat. Artists 

 have availed themselves of this property, to give 

 to surgical and other instruments those degrees of 

 temper which their various uses require. Iron 

 may be alloyed with most of the metals, but these 

 alloys are not much used. 



Wootz is the name given to a kind of steel made 

 in the East Indies, which is of a very superior 

 quality for cutting-instruments. 



