238 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Gay-Lussac expressed his opinion as follows : 



" Iodine is not inflammable ; it cannot even be combined 

 directly with oxygen. I consider it as a simple substance, 

 and I place it .... between sulphur and chlorine, because 

 it has stronger affinities than the former, and weaker than the 

 latter" (Ann. de Chimie, 1814, 91, 8; Klassiker, IV. 5). 



The iodides. Iodine combines directly with a large 

 number of metals, forming compounds which Davy de- 

 scribes as IODIDES. Thus Clement, describing the dis- 

 covery of Courtois, writes : 



" With metallic mercury it forms in the cold, by shaking, 

 a beautiful red powder resembling vermilion. 



" Iodine readily attacks, in the cold, iron, zinc, tin, and 

 antimony. . . . These compounds are soluble in water. 

 Those with lead and silver are not soluble ; the former is of 

 a beautiful yellow colour" (Ann. de Chimie^ 1813, 88, 

 308309). 



Davy found that it formed a salt-like substance by com- 

 bination with the metal potassium : 



" I heated some potassium in a little glass tube, and 

 passed some of the substance in vapour over it ; at the 

 moment the vapour came in contact with the potassium, 

 there was an inflammation and the potassium burnt slowly 

 with a pale blue light. The substance formed by the action 

 of potassium was white, fusible at a red heat, and soluble 

 in water. It had a peculiar acrid taste " (Davy's Works, 

 V. 441). 



Iodine combined with chlorine to form a yellow solid, 

 volatile by heat. Although it did not combine directly with 

 oxygen, a solid oxide was prepared by Davy in 1815 : 



" The compound of oxygen and iodine appears as a white 

 semi-transparent solid ; it has no smell, but a strong 

 astringent sour taste. . . . When strongly heated it decom- 

 poses, undergoing fusion at the moment, and is entirely 

 converted into gaseous matter and iodine, leaving no 



