2 4 o HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



In presence of oxygen the action proceeds to completion 

 oxygen + hydriodic acid -> water + iodine. 



This complete decomposition of the acid is directly contrary 

 to the main course of the action which Berthollet had 

 discovered in chlorine water exposed to sunlight 



oxygen + hydrochloric acid < water + chlorine. 

 The properties of hydriodic acid. Gay-Lussac found that 

 hydriodic acid was prepared more readily by combining iodine 

 with phosphorus and decomposing the iodide with water. 

 In this action water is decomposed ; the oxygen unites with 

 phosphorus to form phosphorous acid, whilst the hydrogen 

 unites with iodine to form hydriodic acid. Gay-Lussac 

 made a careful study of the acid and records its properties 

 as follows : 



" Hydriodic gas decomposes as soon as it comes into 

 contact with mercury ; the surface of the metal becomes 

 covered with a yellow-green substance, which is the iodide 

 of mercury, and if the contact is sufficiently prolonged, or if 

 it is shaken, the hydriodic acid is promptly decomposed ; 

 the iodine combines completely with the metal, and there 

 remains a volume of hydrogen gas which is exactly 

 the half of that of the hydriodic gas. I passed the gas over 

 zinc and potassium, and the product was always hydrogen 

 and an iodide. Thus, according to this analysis . . . there 

 can remain no doubt as to the nature of hydriodic gas. 



" This gas is colourless ; its odour resembles that of hydro- 

 chloric gas ; its taste is very acid ; it contains half its volume 

 of hydrogen, and saturates its own volume of ammonia. 

 Chlorine removes its hydrogen instantly ; a beautiful violet 

 vapour is produced, and hydrochloric gas is formed " (Ann. 

 de Chimie^ 1814, 91, 14 15). 



C. BROMINE. 



Balard (1826) discovers bromine. In a memoir "On a 

 peculiar substance contained in sea-water" (Ann. Chim. 

 Phys., 1826, 32, 337), Balard, a young French chemist, 



