THE SUDDEN COOLING OF PURE IRON. 29 



the writers are rather inclined to believe that the chief cause is probably the 

 state of subdivision the very finely divided particles giving a higher electro- 

 motive force than the fused mass. 31 If this is true, the case is one of great 

 interest because of the magnitude of the effect, corresponding as it does to 

 a tenfold greater solubility of the finest particles. 



In this connection it was deemed worth while to test the electromotive 

 force of iron which had been suddenly cooled or quenched. In this way it 

 might be found whether or not pure iron, when suddenly cooled, could be 

 caught in a condition of internal structure less stable than when cooled 

 slowly. 



Preliminary experiments, to be recounted later in another connection, 

 showed that in order to obtain significant results for this purpose the heat- 

 ing and sudden cooling must occur in the absence of any foreign substance 

 capable of reacting upon the iron ; therefore the metal must be quenched in 

 the absence of any gas or water. 



The apparatus needed was not complex. A stout tube of Berlin porcelain 

 was clamped in a vertical position, closed at both ends by Hempel water- 

 cooled stoppers, and was heated in the middle by an encompassing perforated 

 Fletcher furnace. The temperature was registered by a platinum-rhodium 

 thermopile. The porcelain plate on which the test-pieces of iron were laid 

 was suspended in the middle of the tube by an iron wire on one side and a 

 small silver wire on the other. On the lower Hempel stopper lay a smooth, 

 thick iron plate, and ice-cold water was circulated through the stopper, 

 while the whole lower end of the tube was packed in ice. The apparatus 

 was now evacuated with a Sprengel mercury pump, and when the pressure 

 had fallen to less than 1 mm. the tube was heated. For ten minutes the 

 temperature was kept above i,ooo ; the silver wire was melted and one side 

 of the porcelain plate was dropped. The iron of course fell from this region 

 of high temperature to the cold iron plate. The apparatus was left to cool 

 over night, without the admission of a trace of air. The two pieces of iron 

 when put in had been blackish-gray and very loosely knit together; when 

 taken out they were quite cohesive and of a clear-gray color, exactly 

 resembling iron which had been reduced in hydrogen above 700 . This 

 illustrates the effect of sintering and proves that the original dark color of 

 the iron powder was not due to incomplete reduction, but solely to its state 

 of division. One surface of one of the two pieces was found lying flat on the 

 cold iron plate and this one was immersed in ferrous sulphate and measured. 

 It gave in eight minutes a potential of 0.788, which slowly rose to 0.805 m 

 an hour, and then settled down to constancy at the value of 0.795. 



Ostwald, Zeitschr. fur Phys. Chem., 34, 45 (1900) ; Hulett, ibid., 37, 385 (1901). 



