170 



Prof. D. E. Hughes. 



[Dec. 20, 



by drawing or hammering. Tims a bar of iron drawn or hammered 

 has a peculiar structure, say a fibrous one, which gives a greater 

 mechanical strength in one direction than another. This bar, 

 if thoroughly annealed at high temperatures, becomes homogeneous, 

 and has no longer even traces of its previous strains, provided that 

 there has been no actual mechanical separation into a distinct series 

 of fibres. 



Table I. 



Influence of annealing upon Swedish iron, Sample G. 



Degrees of softness 

 indicated upon the 

 magnetic balance. 

 Wire, hard drawn, as furnished by the makers . . 230° 



Annealed at black heat . . , 255° 



dull red 329° 



bright red . . 438° 



yellow 507° 



„ yellow-white . . 525° 



From the above table, we notice that a regular increase of softness 

 occurs, as the temperature at which it is annealed increases, the 

 maximum being at a point under that of fusion. 



Numerous methods of annealing were tried, the highest results 

 being obtained when the iron or steel was heated as rapidly as 

 possible to a high temperature, and cooled in a neutral surrounding or 

 atmosphere. The facts regarding annealing as pointed out by the 

 measurement of the magnetic capacity of iron, have, no doubt, been in 

 great measure perceived by ordinary mechanical methods ; the results 

 of my own researches may be thus formulated : — 



1. The highest degree of softness in any variety of iron or steel is 

 that obtained by a rapid heating to the highest temperature less than 

 fusion. 



2, The time of gradual cooling required varies directly as the 

 amount of carbon in alloy. 



Thus in chemical pure iron rapid cooling, as in tempering, would 

 not harden it, whilst steel might require several days, even for pieces 

 only 1 millim. diameter. Slow cooling has no injurious effect upon 

 pure iron when cooled in a neutral surrounding, consequently where 

 time is no object, we may employ slow cooling in every case. 



A wire or piece of iron, thoroughly annealed, must not be bent, 

 stretched, hammered, or filed, as the hardening effects of a bend are 

 most remarkable, and the mere cleaning of its surface by sand paper 

 hardens its surface several degrees. 



The following table shows the effect of annealing upon a series of 



