558 TRANSACTIONS OF SECTION B. 



The grey part, although slowly attacked by cold acid, did dissolve, yielding 

 much voluminous silica. The white part was almost inert and only dissolved in 

 strong hydrochloric acid with difficulty, and when the iron was dissolved out the 

 remaining silica was of the dense variety, from which it would appear that the 

 effect of the sulphide is akin to that of sudden quenching. 



It was the micro-structure of the white portion, however, which was of unique 

 interest. On 'heat-tinting' two kinds of hard crystals appeared, one more readily 

 coloured by heating than the other. The more resistant crystals were idiomorphic, 

 and were furnished with their terminal angles, but as they were embedded in the 

 surrounding metal it was impossible to form any exact idea of the crystalline 

 system to which they belong. 



The second order of crystals had evidently solidified at a later period, as their 

 forms were interfered with by those of the idiomorphic type ; they were much like 

 ordinary plates of carbide of iron. The ground mass contained indications of 

 octahedral on fir-tree crystallites and a well-doveloped phosphorus iron eutectic of 

 the honeycomb type. This eutectic was the last to freeze, as it filled the spaces 

 between the plates of the hard crystals. There was no pearlite excepting in the 

 eutectic of phosphorus and iron. We can only tentatively conclude that of the 

 two cementites the idiomorphic crystals contained the greater part of the silicon 

 because of their greater resistance to oxidation and probably consisted of carbo- 

 silicide of iron, with sufficient sulphur in them to make them stable; also that 

 the second crystals were carbide of iron, possibly containing a lesser quantity or 

 no silicide in solid solution. 



A further series of experiments was made on a portion of the same metal. In 

 this case the molten iron was mixed and agitated with free sulphur instead of 

 sulphide of iron, and the metal was at once poured into a sand mould in a thin 

 layer. When cold it was quite white in fracture and had large brilliant cleavage 

 faces. 



It had the following composition : — 



Per cent. 



Combined carbon 2 - 60 



Manganese Trace 



Silicon 6-65 



Sulphur 0-93 



Phosphorus 2'08 



The sulphur had evidently effected concentration of the silicon phosphorus and 

 carbon by removing some of the iron, as sulphide of iron was actually formed and 

 floated on the surface of the iron. It was fractionally dissolved as described in 

 previous cases, and the residue (72 per cent, of the weight of the original metal) 

 was tested and found to contain : — ■ 



Per cent. 



Carbon 292 



Manganese Trace 



Silicon 6-70 



Sulphur . 0-062 



Phosphorus 1'410 



This insoluble fraction evidently consisted of both classes of crystals, together 

 with some phosphide of iron. Efforts were made to separate the crystals by 

 chemical means, but without success. 



On the long and continued action of strong hydrochloric acid a residue was 

 obtained containing a little less carbon and more silicon than were present in the 

 mixture, an indication that the less soluble portion is different from that more 

 soluble. 



The micro-structure was similar to that of the metals of the previous trial, 

 but as the carbon and silicon were higher the carbo-silicide was in greater 

 quantity. It crystallised in long flat plates and not in relatively short idiomorphic 

 crystals. 



It is probable exception may be taken, with some justification, that the 

 sulphur does not simply arrest the decomposition of the cementites, which I have 

 premised primarily form, but may act in some other unknown way. An attempt 

 was therefore made to find out whether they could be obtained by some other 

 method without the aid of sulphur. As it is known that the ternary eutectic 



