PHYSICAL LABORATORY OF GREAT BRITAIN. 351 



doing wnieh it will realize the aims of its founders. The microscopic 

 examination of metals was begun by Sorby in 1864. Since that date 

 many distinguished experimenters, Andrews, Arnold, Ewing, Martens, 

 Osmond, Roberts-Austen, Stead, and others have added much to our 

 knowledge. 1 am indebted to Sir W. Roberts- Austen for the slides 

 which I am about to show you to illustrate some of the points arrived at. 

 Professor Ewing, a year ago, laid before the Royal Institution the results 

 of the experiment of Mr. Rosenhain and himself. This microscopic 

 work has revealed to us the fact that steel must be regarded as a crys- 

 tallized igneous rock. Moreover, it is capable at temperatures far 

 below its melting point of altering its structure completely, and its 

 mechanical and magnetic properties are intimately related to its 

 structure. The chemical constitution of the steel may be unaltered, the 

 amounts of carbon, silicon, manganese, etc., in the different forms 

 remain the same, but the structure changes, and with it the properties 

 of the steel. Figure 1 on Plate II represents sections of the same 

 steel polished and etched after various treatments. a 



The steel is a highly carbonized form, containing 1.5 per cent of 

 carbon. If it be cooled down from the liquid state, the temperature 

 being read by the deflexion of a galvanometer needle in circuit with a 

 thermopile, the galvanometer shows a slowly falling temperature till 

 we reach 1,380° C, when solidification takes place. The changes which 

 now go on take place in solid metal. After a time the temperature 

 again falls until we reach 680°, when there is an evolution of heat; had 

 the steel been free from carbon there would have been evolution of 

 heat at 895° and again at 766°. Now throughout the cooling molecular 

 changes are going on in the steel. By quenching the steel suddenly at 

 any given temperature we can check the change and examine micro- 

 scopically the structure of the steel at the temperature at which it was 

 checked. 



In the figure (Plate II), with the exception of specimen No. 6, the 

 metal has not been heated above 1,050°, over 300 below its melting- 

 point. 



• specimen. 



1. Raised to 1000°. Worked and cooled slowly. Masses of carbide ground work, 



bands of iron and carbide, pearlite structure, 

 l'. Raised to 850° and quickly cooled. Masses disappear. 



3. Raised to 850° and quenched in water. Arcicular structure. Martensite, bard 



steel. 



4. Raised to 1,050° and quenched in iced brine. Martensite and Austenite. 



5. Same cooled in liquid air to — 243°. Much like martensite. 



6. Heated to near melting point, quenched suddenly burnt steel. 



7. Heated to 650° — annealed for a long time at this temperature and slowly c< h (led, 



bands of carbide and pearlite. 



8. Any specimen except 6 heated to 850°, worked and slowly cooled, giving us the 



structure 1. 

 Very marked changes might have been produced in 3 by annealing at 140°. 



