126 PRACTICAL LESSONS IN SCIENCE. 



changes the properties of the 127 parts forming a colorless 

 gas, formula HI. 



Iodine forms an acid with hydrogen and oxygen called iodic 

 acid, formula HIO, analogous to chloric and bromic acid. 



Chlorine, bromine and iodine constitute a well-defined natural 

 group of elements. They each unite with hydrogen in the same 

 proportions, forming acids of similar properties and each form- 

 ing similar salts. Each of these elements forms a powerful acid 

 containing three atoms of oxygen. With nitrogen they each form 

 explosive compounds. At ordinary temperatures chlorine is a 

 gas, bromine a liquid and iodine a solid. Chlorine is yellow, 

 bromine reddish-brown and iodine violet. In general chlorine is 

 more energetic and its compounds more stable, while iodine is 

 least energetic except in its combinations with oxygen. 



FLUORINE is an element, known only in its compounds, of which 

 the most common is the mineral fluor spar. It occurs in vegetable 

 and animal substances and in several other minerals. Fluorine 

 forms an acid with hydrogen analogous to hydrochloric acid, 

 which is very corrosive, attacking glass vigorously, so that it 

 must be collected in lead, rubber, or platinum vessels. It is used 

 for etching glass. The glass is covered with a thin coat of wax, 

 and lines or figures are drawn with a pointed instrument so as 

 to cut through the wax. The glass is then exposed to the action 

 of the acid, and soon the figures drawn are corroded into the 

 glass. Fluorine seems to have affinities with the chlorine group 

 of elements, but so little is known of it that no extensive com- 

 parison can be made. 



