LESSONS IN CHEMISTRY. 125 



element. Some compounds of bromine are used as bleaching and 

 disinfecting agents. Some are used in photography, and bromide 

 of potassium is used extensively in medicine. 



IODINE (I) is also found in seawater. Certain varieties of sea- 

 weed separate the sodium iodide from the seawater and concen- 

 trate it in their tissues. The seaweed is collected, dried and 

 burned. The ashes and other residue are leached with water, 

 which dissolves out the sodium iodide, and this solution yields 

 the iodine of commerce. Iodine is a grayish black crystalline 

 solid of metallic luster. It melts at 225 F. and boils at 356, 

 forming a beautiful violet-colored vapor, which is the heaviest 

 known, being 872 times heavier than air. Drop a piece of iodine 

 as large as a grain of wheat into a test-tube and apply heat; 

 observe the vapor, notice the coating it forms on the tube; apply 

 heat to different parts of the tube and study the effects. Exam- 

 ine the coating for crystals. 



Iodine is soluble in alcohol or ether and is sparingly soluble 

 in water. Iodine colors starch paste a deep blue ; this reaction 

 constitutes the most delicate test of its presence. Take a thin 

 solution of starch paste and add to it a few drops of an aqueous 

 solution of iodine and notice the color. Dip a piece of white pa- 

 per in the paste and suspend it while moist in a bottle or jar on 

 the bottom of which has been placed a small piece of iodine. In 

 a few moments the paper begins to show a blue color, indicating 

 that iodine is volatile at ordinary temperatures. Iodine stains 

 the skin yellow but is not as corrosive as either chlorine or bro- 

 mine. A solution of iodine in alcohol is familiarly known as 

 tincture of iodine and much used as a liniment. Iodide of mer- 

 cury is sometimes used in painting as a pure scarlet; the iodide 

 of lead has a bright golden color; the iodide of silver is yellow 

 and the iodide of potassium is white. The iodide of potassium 

 is extensively used in photography and in medicine. 



Iodine combines with hydrogen forming hydriodic acid, a 

 colorless gas, with a suffocating odor. One part of hydrogen, 

 weight one, combines with one part of a highly colored iodine 

 vapor, weight 127, and the one part discharges the color and 



