524 Transactions. 



blast-furnace slag. Then all of them were heated at one end to 

 a little above 1,000° C. and kept comparatively cool at the other 

 extremity. The temperatures at equal intervals along the bars 

 were measured by a Chatalier pyrometer. After heating until 

 the temperatures were constant each bar was quenched in cold 

 water, ground bright, polished, and etched, and micro-photo- 

 graphs prepared. Fig. 6 shows the result. 



In the case of No. 1 bar (pure iron) there is no apparent 

 change of structure until a temperature of 870° C. is reached. 

 Then reorganization is apparent, the crystals becoming smaller. 



No. 2 Bar (0-2 per cent, carbon). — In proportion as the tem- 

 perature has exceeded 750° C. so the carbon has diffused from 

 the carbide areas (pearlite) into the surrounding ferrite, until 

 at about 1,000° C. the diffusion is complete (martensite). It 

 will be noticed also that the breaking-up of the ferrite-crystals 

 occurs, as before, at the Ar 3 point. 



No. 3 Bar (0-4 per cent, carbon). — Diffusion is complete at 

 about 830° C. 



No. 4 Bar (0-6 per cent, carbon). — Diffusion is complete at 

 770° C. 



No. 5 Bar (pure pearlite (saturated) steel which contains 

 - 9 per cent, carbon). — Diffusion or solid solution is complete at 

 Ar, point, 690° C. 



No. 6 Bar (1*4 per cent, carbon). — The excess of cement- 

 ite shows here. Diffusion is complete at 1000° C. (Ar 3 point). 



On the figure the writer has drawn lines, the loci of the 

 Ar 3 points, for the different percentages of carbon contained in 

 the bars, and it is remarkable to note how nearly this compara- 

 tively rough experiment of Mr. Stead's agrees in each case with 

 the theoretical point of diffusion. 



From the foregoing it will be seen that steel, instead of being 

 the homogeneous material it is popularly supposed to be, in 

 reality partakes more of the nature of a crystalline rock ; and 

 this is especially true of the normal low and medium carbon 

 steels so much used for constructive work and machine details. 

 When the complex nature of its structure is taken into con- 

 sideration it appears probable that such steel may break in 

 more than one way ; and apparently this is the case, for when 

 broken by a gradually applied tensional or repeated or alternated 

 load there are invariably present in the neighbourhood of the 

 fracture the slip planes between the particles first noticed by Pro- 

 fessor Ewen and Mr. Rosenhain, but slip planes have not been 

 found in any case where the fracture has been due to shock. 



That capacity to resist shock is a property distinct from 

 that of resistance to progressive loading is shown by the follow- 

 ing tables of the results of experiments conducted by Mr. Seaton, 



