188 Modern Microscopy 



infusion of liquorice root and tincture of iodine. Very 

 dilute nitric acid and sulphuric acid are also used, but 

 until the student has become thoroughly acquainted with 

 the effects of the above he is not advised to use them. 



Before proceeding further it is advisable to give an out- 

 line respecting the effects of the reagents, also the construc- 

 tion of the metal. 



Steel is viewed as if it were a rock with various con- 

 stituents in it. There are three principal, ones — viz., 

 ferrite, cementite, and pearlite (or sorbite). 



Ferrite. — This is iron free from carbon. It retains a 

 very dull polish, and is not stained by iodine or liquorice. 



To develop the crystalline structure of ferrite a very 

 dilute solution of nitric acid in alcohol should be used. 



Cementite. — This is a very hard substance, and stands 

 in relief after polishing, as above. It is very rarely found 

 in low carbon steels, and is left bright after the polished 

 surface is attacked by iodine. 



Pearlite. — This is a very intimate mixture of ferrite and 

 cementite. If the steel has been allowed to cool slowly 

 from a very high temperature, pearlite assumes a well- 

 defined lamellar structure ; on the contrary, if the metal 

 has been forged or reheated at a very low temperature, 

 pearlite assumes a granular appearance. It is readily acted 

 upon by iodine or liquorice. 



Prom this it will be seen that steel is made up of (1) 

 ferrite and pearlite, (2) pearlite, (3) pearlite and cementite. 

 Other constituents are found in steel after it has undergone 

 certain treatment, but enough has been said to guide the 

 student to make a commencement. 



The reagent can now be applied. This is done by either 

 coating the specimen with some protective varnish, leaving 

 the surface free that is to be acted upon, and immersing 

 the whole in a bath, or a few drops may be applied to the 

 surface, and then carefully spread by means of a glass 

 dipping-rod. 



The solution should be allowed to act for, say, twenty 



