IODINE. 89 



small quantity of this substance which water is 

 capable of dissolving. 



2. Gay-Lussac found that when iodine and 

 zinc are heated together in a vessel hermetically 

 sealed, and containing a sufficient quantity of 

 water, a colourless solution is obtained, consist- 

 ing of hydriodate of zinc. In this case he found 

 that 100 parts of iodine combine with 26*225 

 parts of zinc.* Now 26-225 : 100 : : 4-25 fan 

 atom of zinc) : 16*20. According to this experi- 

 ment, 16*20 denotes the atomic weight of iodine. 

 Gay-Lussac himself deduces from his experi- 

 ment, that the atomic weight of iodine is 15*621 ; 

 because he conceived that 26*225 of zinc com- 

 bine with 6*402 oxygen, whereas the quantity of 

 oxygen with which this weight of zinc combines, 

 when oxydized, is only 6*170 parts. 



This experiment of Gay-Lussac was repeated 

 by Dr. Prout, who found that 50 parts of iodine 

 combine with 12*9 parts of zinc.t According to 

 this experiment, the atomic weight of iodine 

 turns out 16*473, which is a little higher than 

 that stated by Gay-Lussac. 



3. I have repeated this experiment a good Method of 

 many times, but I did not obtain constant re- ingtatom 

 suits, partly from the difficulty of preventing the fiodine * 

 loss of some of the iodine, and partly because 



the zinc was never perfectly pure. I therefore 

 had recourse to another method, which I shall 



* Aun. de Chim. XCI. 2-4. f Annals of Philosophy, VI. 323. 



