18 



HOW CROPS GROW. 



chlorate. Exposed to heat, this body melts, and present- 

 ly evolves oxygen in great abundance. 



EXP. 4. The following figure illustrates the apparatus employed for 

 preparing and collecting this gas. 



A tube of difficultly fusible glass, 8 inches long and J inch wide, con- 

 tains the red oxide of mercury or potassium chlorate.* To its mouth is 

 connected, air-tight, by a cork, a narrow tube, the free extremity of 

 which passes under the shelf of a tub nearly filled with water. The 

 shelf has, beneath, a funnel-shaped cavity opening above by a narrow 

 orifice, over which a bottle filled with water is inverted. Heat being 



Fig. 2. 



applied to the wide tube, the common air it contains is first expelled, 

 and presently, oxygen bubbles rapidly into the bottle and displaces 

 the water. When the bottle is full, it may be corked and set aside, and 

 its place supplied by another. Fill four pint bottles with the gas, and 

 set them aside with their mouths in tumblers of water. From one 

 ounce of potassium chlorate about a gallon of oxygen gas may be thus 

 obtained, which is not quite pure at first, but becomes nearly so on 

 standing over water for some hours. When the escape of gas becomes 

 slow and cannot be quickened by inn-eased heat, remove the delivery- 

 tube from the water, to prevent the latter receding and breaking the 

 apparatus. 



As this gas makes no peculiar impressions on the senses, 



*The potassium chlorate is best mixed with about one-quarter its 

 weight Of powdered black oxide of manganese, as this facilitates the 

 preparation, and renders the heat of a common alcohol lamp sufficient. 



