PRESIDENTIAL ADDRESS. 561 



In white irons containing 3 per cent, carbon and under 2 per cent, silicon, after 

 the primary austenite has fallen out of solution carrying practically all the 

 silicon, it is not iron-iron-carbide entectic which forms, but independent plates 

 of cementite, or carbide of iron, and after these have crystallised and the 

 residual mother liquor has arrived at the composition of the ternary iron-carbon- 

 phosphorus eutectic, the latter solidifies at 945° C. 



7. In Cleveland irons which become grey on cooling, and in which there is no 

 primary austenite, the same iron-carbon-phosphorus eutectic is the only eutectic 

 to form during cooling, and, instead of a ternary iron-carbon-silicon eutectic. 

 two independent cementites crystallise — one a silico-carbide, and the other 

 carbide of iron possibly containing a little silicide in solid solution. The micro- 

 examination of the cold alloys, to which a little sulphur had previously been 

 added when the metals were melted, led to the conclusion that it is the carbo- 

 silico-cementite which primarily crystallises. 



8. There is evidence that the primary carbo-silicides are exceedingly unstable 

 and are the first to decompose into graphite and silico-austenite. 



9. In the absence of any sensible quantity of phosphorus, two cementites form 

 — one the silico-carbide cementite, the other the carbide cementite — and these 

 crystallise together as a eutectic mixture. 



10. The exact composition of the two cementites has not yet been determined, 

 as no chemical method has been found for their isolation. 



11. It is evident -that it is the exceedingly unstable character of the silico- 

 carbides which is responsible for the greyness of commercial metals rich in 

 silicon and low in sulphur. 



12. Silicide of iron when heated at 1000° C. with pure white iron free from 

 silicon effects the decomposition of the carbide of the white iron. Based on this 

 observation the hypothesis seems justifiable, in cases where all the silicon present 

 in hypo-eutectic alloys crystallises out with the primary austenite, that after the 

 carbide has solidified, diffusion of the silicide follows, and this leads to the 

 decomposition of the carbide of iron into graphite and iron. 



13. Many of the results arrived at by chemical analysis support the hypo- 

 thetical conclusions of Gontermann, who depended mainly on data obtained by 

 thermal methods of treatment. 



In conclusion, it will be clear from what I have stated that there are many 

 gaps yet to be filled. I hope that the knowledge of this fact will lead others 

 to follow up the research, which, in its present stage, is far from complete. 



Note. — The terms carbide cementite and silico cementite are used tentatively. 

 Strictly, there is only one cementite and that is F,C. 



FRIDAY, SEPTEMBER 2. 



Joint Meeting with Section A. 

 Discussion on Combustion. — See Reports, p. 501. 



MONDAY, SEPTEMBER 5. 



Joint Discussion with Sections I and K on the Biochemistry of 

 Respiration. — See p. 762. 



The following Paper was then read : — 



On the Influence of the Pressure, Humidity, and Temperature of the Atmo- 

 sphere on the rate of Metabolism in Animals. By Wm. Thomson. 



Briefly stated, a rise in the barometric pressure or a rise in the humidity of 

 the atmosphere produces a lowering in the percentage of CO, in the exhaled air, 



