250 Mr. G. A. Hemsalech : Excitation of Spectra of Carbon, 

 was protected. The upper surface, which naturally loses 

 heat more rapidly, gave, with a potential drop of 5'63 V ^ > — 8 

 only about 2250° C. These figures emphasize the great 

 importance of the protecting layer of carborundum, and it 

 deserves to be mentioned that some of the most important 

 results achieved in the course of the present investigation 

 were directly due to this precaution. 



§ 4. Luminous phenomena observed in the vicinity of the 

 electrically heated plate of graphite. 



Observations were made by forming an image (about twice 

 actual size) of the heated plate upon a white cardboard screen. 

 This plan proved not only very convenient, but it also had 

 the further advantage over direct observation through dark 

 glasses, of showing the phenomena in their true natural 

 colours, — a most helpful adjunct for interpreting the meaning, 

 of the luminous effects displayed. 



As the temperature of the plate is gradually raised,, 

 yellowish vapours begin to form along the undersurface 

 and a continuous stream of similar vapours is seen to rise 

 upwards from above the plate. At temperatures of from 

 2300° to 2500° C, the coloration of the vapours beneath the 

 plate changes to bluish grey and, furthermore, they present 

 now a, sharply-defined outline as sketched in fig. 3. In the 

 region above the plate, where the temperature is much lower,, 

 the colour of the rising vapours continues yellow, with here 

 and there a greyish streamer or patch. In the neighbour- 

 hood of the plate, principally just above it, are also seen 

 red-coloured regions. All these colour effects are due, no 

 doubt, to the various light radiations emitted under the pre- 

 vailing temperature conditions, by the vapours driven out 

 from the graphite plate and the carborundum. 



In order to account for the sharp demarcation of the 

 incandescent vapours beneath the plate two factors must be 

 taken into consideration — namely, the continuous effusion, 

 from the undersurface, of vapour which is being forced 

 downwards into the protected space, and the upward 

 draught of air, as explained in § 2. At the boundary sur- 

 face of the protected space the hot vapour comes into contact 

 with the air current, is cooled with consequent changes in 

 its radiating properties, and is then immediately carried 

 away upwards. Since it is reasonable to assume that, whilst 

 the temperature of the plate remains constant, both the 

 quantity of vapour passing through the protected space and 



