252 Professor Arthur Smithells on 



Incandescent Iodine Vapour. 



Having discussed the above question with Sir G. Gr. Stokes, 

 I tried, at his suggestion, the experiment of heating iodine 

 vapour. This, being strongly absorptive at ordinary tempera- 

 tures, might be expected to emit visible radiations at com- 

 paratively low temperatures. 



The experiment was made by filling the bowl of a " church- 

 warden " tobacco-pipe with iodine and closing with a lute of 

 plaster of Paris. The stem was laid along a Fletcher tube- 

 furnace and strongly heated. The bowl of the pipe, which 

 protruded from one end, was heated with the flame of a Bunsen- 

 burner. The tip of the stem, which protruded about one inch 

 from the other end, was shielded from stray light and viewed 

 against a background of black velvet. When the stem was 

 heated to such temperature as could be obtained with the 

 unforced draught of air, the issuing stream of iodine vapour 

 was not luminous; but when the furnace was fed with a blast 

 of air and the temperature sufficient to soften the pipe, the 

 iodine became luminous and the vapour presented the appear- 

 ance of a pointed yellowish flame about an inch in length. 

 A similar appearance was obtained when a glazed porcelain 

 tube was substituted for the pipe-stem, so as to exclude all 

 possibility of furnace-gases diffusing into the interior. My 

 attention was subsequently drawn to the fact that the effect 

 had been already described by Salet (Pogg. Ann. cxlvii. 

 p. 319) and had been achieved in a simpler way. 



The experiment may be best performed as follows: — A piece 

 of hard glass tube about half inch diameter and five inches 

 long and closed at one end is suspended horizontally by 

 platinum wires from the ring of a retort-stand and heated 

 along its whole length by the flame of a flat Bunsen-burner. 

 This should be done in the dark, and the burner removed as 

 soon as the glass is seen to glow. The tube is allowed to 

 cool till it has just ceased to glow, and then a long piece 

 of platinum-foil bent into a trough and filled with iodine is 

 passed into the tube and the iodine tipped out. The tube is 

 immediately filled with the glowing vapour of iodine. This 

 can be made to glow still more brightly by a reapplication 

 of the flame. How much of the luminosity is due to the 

 iodine and how much to the hot glass may be seen at any 

 moment by blowing out the iodine vapour by a puff of air. 

 The effect is very marked, and there seems no room to 

 doubt that the glowing is due to hot iodine vapour. The 

 lowness of the temperature seems to forbid the suggestion 

 that the glow may be due to combination of the iodine with 



