Line Spectrum emitted by Iron Vapour. 291 



in sucli a way as to reduce the contact surfaces with the main 

 plate to a few sharp points only, thus preventing any large 

 currents from passing through the plates a, b, a'. Further, 

 the plates a and a 1 are inclined at an angle of 60° or less, so 

 that any potential gradient existing between a and a' would 

 decrease rapidly in value on passing from the bottom of the 

 furnace upwards. Two observation-tubes of carbon, each 

 having an internal diameter of 14 mm. and a length of about 

 4 inches, are placed in a line, one in front and the other 

 behind the furnace, as indicated by the dotted circle. Obser- 

 vations are made through these tubes, which afford an un- 

 interrupted view of the metal vapours in the furnace. The 

 whole space both beneath and above the main plate, furnace, 

 and observation-tubes is filled up with carborundum-powder 

 to a depth of at least two inches all round, so that this 

 furnace is as efficiently protected as was the tube-furnace in 

 a former experiment. The metal to be examined was laid 

 on the bare portion of the plate AB comprised between the 

 inclined plates a and a'. The plate was heated by means of 

 direct current of over 300 amperes. As the temperature 

 rose luminous vapours from the walls and carbon particles 

 caused by the disintegration of the graphite gave out a strong 

 continuous spectrum on which were visible the absorption- 

 lines of Na, Ca, and Sr. When iron was boiled in this 

 furnace the interior emitted a brilliant light, but at no time, 

 even up to the burning through of the plate at 0, was there 

 observed any trace of an emission spectrum of this element, 

 such as was beheld in the tube-furnace at 2700°. Nor were 

 the Swan bands ever seen with this furnace. 



Similar negative results were obtained with copper. 



"With thallium, however, the green line was observed first 

 as an emission-line and then, at the higher temperatures, as an 

 absorption-line. It is, however, doubtful whether in this case 

 the emission was due to purely thermal excitation ; it was 

 indeed found that the metal, already at lower temperatures, 

 rapidly formed a compound which adhered to the furnace 

 walls in flaky masses; and it seems to me more probable that 

 it was the action of heat on this compound which caused the 

 emission of the green line : the emission would therefore 

 have been due to thermo-chemical excitation. 



§ 7. Observations on the Spectrum Emission of Metal Vapours 

 under the Influence of an .Electric Field by Means of a New 

 Type of Electric Furnace. 



Having failed to excite the line emission of the vapours of 

 iron and copper by purely thermal actions, it was of course 



