xii THE HALOGENS 253 



air-tight by leaden washers, p, screwed down by brass caps, E. 

 Hydrogen was liberated at the negative and fluorine at the 

 positive electrode. It is a light-yellow gas, condensing to a 

 liquid at - 187 C., and crystallising to a pale yellow solid at 

 - 233 C. It is a very active gas, unites explosively with hydrogen, 

 and at once decomposes water, forming hydrofluoric acid, 

 ignites charcoal, combines vigorously with phosphorus, liberates 

 chlorine from potassium chloride, and unites directly with 

 nearly all the metals, but has no marked action on dry glass. 



B. IODINE 



Iodine, discovered by Courtois in 1811 in the ashes of 

 seaweed, resembles chlorine in its properties, and in its resist- 

 ance to all efforts to effect its decomposition. It was examined 

 by Clement, by Gay-Lussac, and by Davy, four memoirs on 

 iodine being published within a fortnight at the close of 1813. 

 The beautiful violet vapour is not decomposed by passing 

 through a red-hot tube, alone or with oxygen, nor by the action 

 of white-hot charcoal. But it unites directly with the metals to 

 form fusible iodides. It also unites with sulphur and phos- 

 phorus ; with chlorine it forms a solid yellow chloride Id. It 

 unites directly at a red heat with hydrogen to form gaseous 

 hydrogen iodide or hydriodic acid, but this action is reversible 

 as the acid is decomposed partially by the same treatment : 



(Hydrogen 

 iodide.) 



Hydriodic acid is prepared more easily by the action of water 

 on iodide of phosphorus : 



2PI 3 + 6H 2 O -> 2H 3 PO 3 -t-6HI, 



(Phosphorous (Hydriodic 

 acid.) acid.) 



or by the action of sulphuretted hydrogen on iodine suspended 

 in water : 



SH,+ I 2 -> S + 2HI. 



The gas is intensely soluble in water, giving rise to a fuming 



