2660 



Journal of Applied Microscopy 



temoer them or let them down, by heating to a lower temperature than was 

 employed in hardening. 



"In the case of flat chisels, containing about 1 per cent, of carbon, the 

 quenched tools are carefully reheated to about 2T0°C, and again quenched, the 

 right temperature being known by a polished surface showing a brownish-yellow 

 oxidation film. 



" The surface of a piece of one of the chisels, near the cutting edge, was care- 

 fully polished by means of successively fine emery papers and finally with wet 

 rouge on a smooth cloth. After etching and examination with the microscope, no 



Fig. 10. 



structure could be distinguished, and this led to the conclusion that the superfi- 

 cial film was homogeneous, and probably broke with a ronchoidal fracture. Then 

 on another piece the surface was ground away, until the interior was reached, 

 care being taken to keep the piece cold during grinding. After preparation and 

 examination, it was found that it possessed a cellular structure (Fig. 10 ; mag- 

 nification 220 diameters), the cells being composed of hardenite enclosing many 

 very small globules of cementite ; and the reticulated walls, of larger globules 

 of cementite ; the latter, while not touching, are so near to each other that they 

 appear as continuous meshes under a low power." 



The absence of distinguishable structure on what I have designated the 



