the Radiation of Heated Gases. 463 



The dark bands, characteristic of cooler iodine vapour, were 

 seen to be unchanged, and there was no sign of any continuous 

 absorption. Thus at the temperature of the experiment 

 iodine emits a continuous spectrum, and does not emit only 

 those rays which it absorbs. 



The question whether iodine is the only element having 

 this property of glowing with a continuous spectrum at com- 

 paratively low temperatures was next investigated. A series 

 of experiments with the allied element bromine immediately 

 showed that this was not so, for the vapour of this substance 

 was also found to glow brightly under similar circumstances. 



The bromine glow seems indeed to be quite as conspicuous 

 as that of iodine: the spectrum also was found to be perfectly 

 continuous. 



It was then thought that chlorine, being also a coloured gas 

 and allied to the others, would probably be found to emit 

 light in the same way. Accordingly arrangements were 

 made for generating and thoroughly drying this gas and 

 heating it in a glass tube, as in the foregoing experiments. 

 The first attempts to see the glow were made, as before, from 

 outside the tube ; but they led to negative, or, at the most, 

 very doubtful results. A very slight alteration in the mode of 

 viewing the heated gas soon, however, revealed a distinct and 

 unmistakable, though faint, luminosity. The experiment was 

 arranged in the following way : — Chlorine from a generating 

 flask, after being freed from HC1 and dried by passing slowly 

 through a long tube of calcium chloride, was led into a 

 straight piece of combustion-tube of about 7 mm. bore, 

 heated strongly in the middle. This tube connected on to a 

 larger glass tube containing a total -reflexion prism placed in 

 line with the central axis of the heated tube : this enabled 

 one to observe the heated gas " end on " without the interpo- 

 sition of the red-hot glass, and a dark background was 

 obtained by covering up the further end of the combustion- 

 tube with opaque material. 



In this way observations were made, first, w ith common air 

 only passed through the apparatus, and afterwards with dried 

 chlorine, about 5 cm. of the tube being kept meanwhile at 

 the highest temperature attainable with an ordinary Bunsen 

 burner. With air, the centre of the tube remained perfectly 

 dark, and no trace of glowing solid matter in the form of dust 

 could be seen. With chlorine, on the other hand, a faint 

 greenish glow gradually filled up the previously dark central 

 bore of the tube, appearing .first on the lower or hottest part. 

 The light, although brighter on the lower side, appeared per- 

 fectly uniform in texture so to speak, unlike the glow 

 produced by incandescent solid particles, which can be seen 



