IODINE. 91 



5 being the atomic weight of potassium, and the 

 iodide being a neutral compound, it is obvious 

 that 15*5 must be the weight of an atom of io- 

 dine. 



The above experiment was repeated, substitu- iodide of 

 ting 19'7<5 grains of iodide of zinc, for the 20%5 

 grains of iodide of potassium. When solutions, 

 containing respectively 19*7^ grains of iodide of 

 zinc, and 20*75 of nitrate of lead, were mixed to- 

 gether, I found it necessary to evaporate the 

 mixture to dryness by means of a very gentle 

 heat. Water being poured upon the yellow re- 

 sidue, was digested on it for some time. It was 

 then tested with sulphate of soda, and with ni- 

 trate of lead, without being in the least affected 

 by either of these re-agents, showing that it con- 

 tained no sensible quantity of lead, or of iodine. 

 Unless the original mixture of the two salts be 

 evaporated to dryness, sulphate of soda is very 

 apt to indicate the presence of lead and if the 

 evaporation be carried too far, or too great a heat 

 be applied to the residual matter, the water 

 poured upon it will acquire a yellow colour, and 

 will contain distinct traces of iodine. If this be 

 avoided, the liquid will contain simply a solution 

 of nitrate of zinc. 



From this experiment, which I have made se- 

 veral times successfully, it is obvious that 19'7^ 

 grains of iodide of zinc contain just an atom of 

 iodine, or the quantity necessary to unite with 



