322 



Mr. John Allen Harker 



[Feb. 9, 



months or so ; it was then found to be quite translucent. The 

 possibiHty of constructing in such a way refractory gas-tight mate- 

 rials at once suggested itself, and we proceeded to manufacture 

 " pottery " at high temperatures. Great difficulties have been en- 

 countered in the experiments. Whereas, for example, the potter in 

 baking his wares at temperatures up to 1300° C. looks for a shrink- 

 age of 5 per cent, or so, we were confronted with a shrinkage of 

 37 per cent. For the purposes of the fritting we employed carbon- 

 tube furnaces of one of the types mentioned above. Now it some- 

 times happened that the outer surface of the zirconia tubes, instead 



GR/\PHITE SP 



Fig, 3. — Carbon-Tube Furnace with Graphite Spiral Heater. 



of having the white and hard appearance of the rest, was found to 

 be carburised and crumbly after baking. The action was not merely 

 superficial, but extended to an appreciable depth. On the other hand 

 the inner surface of the tube, though freely open to the furnace 

 . atmosphere, was much less affected. The blackening occurred to a 

 much less extent if the tube was shielded. It seemed as though 

 particles, possibly electrified, were shot olT from the carbon walls of 

 the furnace across a space of some 5 or more mms. into the material 

 of the refractory tubes. Dr. G. W. C. Kaye and I were led to 

 investigate the cause of these phenomena, and yesterday we gave an 

 account of some of the results to the Royal Society. I propose to 



