34 HOW CROPS GROW. 
ash. Exposed to heat, this body melts, and presently 
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 14 inch wide, con- 
tains the oxide of mercury or chlorate of potash.* To its mouth is con- 
nected, 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 saucer-shaped cavity opening above by a narrow orifice, over 
which a bottle filled with water is inverted. Heat being 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 chlorate of pot- 
ash 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 increased heat, remove the delivery-tube from the water, 
to prevent the latter receding and breaking the apparatus. 
* The chlorate of potash is best mixed with about one-quarter its weight of 
powdered black oxide of manganese, as this facilitates the preparation, and ren- 
ders the heat of a common spirit lamp sufficicnt. 
