236 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Davy, writing in 1814, says : 



"This substance was accidentally discovered about two 

 years ago by M. Courtois, a manufacturer of saltpetre at 

 Paris. In his process for procuring soda from the ashes of 

 sea-weeds, he found the metallic vessels much corroded ; and 

 in searching for the cause of this effect, he made the dis- 

 covery. The substance is procured from the ashes after the 

 extraction of the carbonate of soda, with great facility, and 

 merely by the action of sulphuric acid : when the acid is 

 concentrated, so as to produce much heat, the substance 

 appears as a vapour of a beautiful violet colour, which 

 condenses in crystals having the colour and the lustre of 

 plumbago " (" On a New Substance which becomes a 

 violet-coloured Gas by Heat," Davy's Works , V. 437). 



The properties of iodine. The new substance which 

 Courtois had discovered was described by Desormes and 

 Clement, at a meeting of the Paris Academy of Sciences on 

 November 29, 1813. Their paper was not published in 

 the Memoirs of the Institute, but a summary was given in 

 the Annales de Chimie under Courtois's name. The 

 properties of iodine are there described as follows : 



" The new substance, which has been called iodine on 

 account of the beautiful violet colour of its vapour, has the 

 appearance of a metal. Its specific gravity is about 

 four times that of water. It is very volatile ; its odour is 

 analogous to that of [chlorine] ; it imparts a red-brown stain 

 to paper and to the hands, but this disappears after a short 

 time ; it is neither acid nor alkaline ; on putting it into a 

 retort and heating, it vaporises at a very gentle heat, about 

 75 C. It boils under water and produces a magnificent 

 violet vapour ; when sublimed in considerable quantity, 

 large, brilliant plates are produced, but these are not 

 massive ; it is little soluble in water, more in alcohol and 

 much in ether. 



A red heat does not change the nature of iodine ; it 

 traverses a red-hot porcelain tube without alteration. 



It is the same with iodine vapour in oxygen ; it is not 

 changed at all by a red heat. The violet vapour escapes 



