60 CHEMISTRY. 



water, and add a very little potassium iodide. Now steep 

 some slips of paper in the mucilage thus prepared, and you 

 have a test-paper for ozone. Place a strip of this paper in 

 the flask containing ozone prepared by phosphorus, and 

 it will soon turn blue, owing to the action of the ozone. 

 The explanation is this : Ozone first decomposes the potas- 

 sium iodide, setting iodine free ; now free iodine forms a 

 blue substance with starch, called iodide of starch, and 

 hence the color produced. Ordinary oxygen will not act 

 thus. Traces of ozone are found in the atmosphere, partic- 

 ularly in the country, and it is tested for with this same 

 iodine-starch paper. 



Ozone has strong bleaching properties, and advantage 

 has been taken of this to bleach sugar on a large scale, the 

 ozone being formed by electricity. 



65. Nature of Ozone. Exactly how phosphorus or elec- 

 tricity act in converting oxygen into ozone is not under- 

 stood by chemists. But it has apparently to do with the 

 question of the arrangement of atoms, for the mere arrange- 

 ment of atoms in the molecule of a substance has much to 

 do with the production of the various qualities presented 

 by different substances. Thus in oxygen we have two 

 atoms in one molecule, arranged thus, O=O, the two lines 

 indicating the supposed points of union ; but in ozone 

 we have three atoms of oxygen to a molecule, and ar- 

 ranged thus, / \ You will meet with other cases of allo- 



tropism, as this is called, where a fuller explanation will be 

 attempted. 



QUESTIONS. 



45. Of what does air consist? 46. What is the most abundant of ele- 

 ments ? 47. Describe one way of obtaining oxygen. Write the equation 

 and explain its significance. 48. What makes this method of making ox- 



