PETROGRAPHY AND METALLOGRAPHY. 



167 



homogeneous substance made up of ciystalline needles 



easily distinguished from any other constituents of steel. 



The exact amount of carbon in steel must be deter- 



Fic:. 60. — Microscopic Appearance as Steel with j..5% Carbon 

 (Peaelytf and Cementite). (After Sauveur.) 



mined by chemical methods; but the presence of the 

 mineral forms we have discussed and the manner in 

 which they are associated can only be ascertained under 

 the microscope. Furthermore, the crystals present may 

 be abnormally arranged, being perhaps not closely 

 packed or so large as to produce incipient cleavage planes. 



