34 



HOW CROPS GROW. 



ash. Exposed to heat, this body melts, and presently 

 evolves oxygen in great abundance. 



Exp. 4.— The following fiuTirc illustrates the apparatus employed for 

 preparinij and collecting this gas. 



A tube of difficultly fusible ^'lass, 8 inches Ion;? and J^ inch wide, con- 

 tains the oxide of mercury or chlorate of potash.* To its mouth is con- 

 nected, Mir-tisht, 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 sauc'cr-shaped cavity opening above by a narrow orifice, over 

 which a bottle filled with water is inverted. Heat being applied to the 



Fig. 3. 



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 corlicd 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 ou standing over water for 

 some hours. "When the escape of gas becomes slow and cannot be 

 quickened by increased heat, remove tlie delivery-tube fmm the water, 

 to prevent the latter receding and breaking the apparatus. 



* The chlorate of potash is best mixed with about one-qnarter its weight of 

 powdered black oxide of manganese, as thi? facilitates the preparation, and ren- 

 ders the heat of a common spirit lamp sufEcicnt. 



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