ZOOLOGY AND BOTANY. MICROSCOPY, ETC. HI 



as an alternative method of developing the structure existing at high 

 temperatures. A steel containing 1 p.c. carbon appeared to be very 

 slightly decarburized by prolonged heating in a vacuum ; a very thin 

 decarburized skin was formed at temperatures below Acl, but not at tem- 

 peratures above that critical point. Probably at the higher temperatures 

 the carbide of iron on the surface, losing carbon by volatilization, is re- 

 placed by carbide diffusing from the deeper layers ; no such diffusion 

 takes place at low^er temperatures. 



