PRESIDENTIAL ADDRESS. 559 



of iron, phosphorus, and carbon melts at about 945° C, it appeared probable 

 that if silicon in small quantity were to be melted with an iron-carbon-phosphorus 

 alloy very rich in phosphorus the two kinds of cementites would fall out of 

 solution at a lower temperature, and would probably not decompose into graphite 

 and silico-austenite in cooling down after their formation. To ascertain whether 

 or not this would be the case, a fusible iron-phosphorus-carbon alloy containing 

 more than the eutectic proportion of carbon was made. It had the following 

 composition : — 



Per cent. 



Iron 91-89 



Phosphorus . . ........ 5*37 



Carbon 262 



Silicon, &c ' . . . . 0-10 



Sulphur (J-02 



100-00 



Four hundred grams were melted with sufficient silicon alloy to yield in the 

 mixture : — - 



Per cent. 



Carbon 2'4 



Phosphorus 5-0 



Silicon 2"90 



Sulphur 002 



When melted a portion of it was cast in a sand mould, the remainder was, 

 allowed to cool in the crucible. 



When cold, that cooled in the crucible was quite grey, whilst the portion cooled 

 in sand was white at the lower part and grey on the top part of the casting, 

 results which proved that the alloy was very unstable and that decomposition of 

 the lower part of the casting was arrested by the slight chilling effect of the 

 cold sand. 



On microscopic examination of the white portion the ground mass was found 

 to consist of the binary phosphorus iron eutectic, whilst two different cementites 

 were- embedded in it, one much more rapidly coloured on ' heat-tinting ' than the 

 other. The colours of the constituents of the properly heated and polished metal 

 were as follows : — 



Cementite (a) White 



(b) Red 



Phosphide of iron Purple 



Iron pearlite crystallites Grey 



The part which broke with a grey fracture consisted of octahedral crystallites 

 of silico-pearlite, the binary phosphorus iron eutectic, and undecomposed (red) 

 cementite crystals, but there was a complete absence of the (white) cementite 

 crystals. Graphite was also present in exceedingly fine plates, resembling what 

 is known as temper graphite. 



The evidence here is conclusive that even in the absence of sulphur : — 



(1) Two cementites had formed. 



(2) That one cementite is much more unstable than the other variety, 

 and decomposes in advance into silico-austenite and graphite. 



Having proved that two different kinds of cementite do actually form and 

 crystallise in the phosphorus eutectic it remained to ascertain in what way these 

 crystallise in the absence of the phosphorus eutectic. 



For this purpose two hypo-eutectic alloys were prepared without any phos- 

 phorus, but with sufficient sulphide of iron to check the decomposition of the 

 carbides. 



They contained : — ■ 



(1) (2) 



Per cent. Per cent. 



Carbon 2-40 2-10 



Silicon 3-17 7-10 



Sulphur 1-21 0-82 



Phosphorus 0-02 0"02 



These when cold after casting in sand broke with white fractures. 



o o 2 



