xii THE HALOGENS 239 



residuum whatever. . . . The gas formed is found to be 

 pure oxygen, and the solid sublimate produced is pure 

 iodine" (Davy's Works, V. 494). 



Clement also records (Ann. de Chimie, 1813, 88, 509) 

 the formation by the action of ammonia on iodine of an 

 explosive black powder, which Gay-Lussac (Ann. de Chimie, 

 1814, 91, 29 ; Klassiker, IV. 15) recognised as an iodide 

 of nitrogen. 



Hydriodic acid. De'sormes and Clement found that 

 iodine (like chlorine) is capable of uniting directly with 

 hydrogen to form an acid : 



" Iodine is not acted on by passing it over red hot 

 charcoal ; but hydrogen effects a complete change in the 

 appearance of this substance. 



" If a mixture of hydrogen with dry or moist iodine 

 vapour is passed through the red-hot tube, the violet colour 

 disappears, scarcely any trace of it can be seen, and a 

 colourless gas is collected, of which one part is promptly 

 absorbed by water and the other part is pure hydrogen. 



" The water in which the soluble gas was collected was 

 very acid ; it acquired a red colour and became perceptibly 

 warm" (Ann. de Chimie, 1813, 88, 306 307). 



This experiment was repeated by Gay-Lussac, who made 

 the further discovery, that 



"Hydriodic acid gas is partially decomposed at a red 

 heat; the decomposition is complete if it is mixed with 

 oxygen ; water is formed, and iodine reappears. On the 

 other hand, I have found that on passing water and iodine 

 vapour through a porcelain tube at a red heat, there was no 

 decomposition. There is here a great difference between 

 iodine and chlorine, for the latter removes hydrogen from 

 oxygen ; but there is also a resemblance between sulphur 

 and iodine, since oxygen removes the hydrogen from both " 

 (Ann. de Chimie, 1814, 91, 18). 



The partial decomposition of hydriodic acid by heat is 

 an excellent example of a balanced action 



hydriodic acid ^ hydrogen + iodine. 



