i8 



THE ELEMENTS OF PLANT FOOD 



Note. Ozone is a more active form of oxygen than oro!inary 

 oxygen. The ordinary form of oxygen has two atoms in each mole- 

 cule, being expressed as O 2 , while ozone is expressed O 3 . Ozone 

 is formed by the discharge of an electric machine in the presence of 

 moisture ; it can be detected by its pungent odor. It is formed in 

 large quantities by lightning flashes. This peculiar form of oxygen is 

 one of the most powerful oxidizing agents known. It is present in 

 pure country air and is absent in the atmosphere in and about large 

 cities. There is probably so much of impurity in the air of the city 

 that the ozone gives up one of its atoms in the oxidizing process and is 

 thus transformed into O 2 , or ordinary oxygen. The bleaching of 

 clothes laid out on the grass is probably hastened by the ozone in the 

 air. Putrid meat thrust into a jar of ozone very soon loses its bad 

 odor, because of the activity of this gas. 



HYDROGEN (H) 



Description and Occurrence. Hydrogen is the 

 lightest substance known, being one sixteenth as heavy 



Thistle 

 Tlibe 



FIG. 4. Preparing Hydrogen. 



as O and two twenty-ninths as heavy as air. On account 

 of its lightness H is used for inflating small balloons. 



