IODIDES AND BROMIDES. 301 



millionth part of this element. By the substitution of 

 bromide of potassium in the experiment, an orange 

 color is produced. 



How is this 757. TEST FOR CHLORINE AND IODINE. 



experiment rp^e exper i men t ma y also be made by 



employed as a . ' . 



test for chlo- moistening a slip of paper with starch and 

 iodide of potassium, and holding it in an 

 atmosphere containing a little chlorine gas. 

 An extremely small quantity of chlorine is 

 thus indicated, and the prepared paper thus 

 becomes a test for chlorine. Such paper is 

 also used to show the presence of ozone in 

 the air. 



758. CHANGE OF COLOR BY HEAT. By 



What is said . . . ' 



of the iodide of mixing solutions of iodides of potassium 

 and corrosive sublimate or chloride of mer- 

 cury, a beautiful scarlet iodide of mercury is produced. 

 On heating the dried precipitate it becomes yellow. 

 The experiment is best made with two watch glasses. 

 The iodide is heated in the lower one and collects by 

 sublimation, with changed color, in the upper. 



What effect is 759. CHANGE OF COLOR BY TOUCH. 



produced by On touching the yellow incrustation with 



touching the . . ,. ,, , . , 



yellow incrus- the point of a needle, it is immediately 

 tation? stained scarlet at the point of contact. The 



color gradually spreads, as if it were a contagious dis- 

 ease, through the whole mass, until every particle has 

 regained its original scarlet. This experiment fur- 

 nishes a very remarkable instance of change of an im- 

 portant property without change of composition. As 



