THE ELEMENTS OF PLANT FOOD 15 



the air, it is difficult to separate it from the nitrogen 

 with which it is thoroughly mixed. It may be more 

 easily obtained from one of its compounds by the appli- 

 cation of heat. Mercuric oxide (HgO) may be used for 

 this purpose. Put about a tablespoonful of mercuric 

 oxide in an ignition tube. Fit the tube with a perforated 

 cork and a delivery tube leading underneath a shelf 

 made by a pan (Fig. i) with an opening in its side and 

 another in its bottom. Invert the pan in another and 

 deeper pan. Pour in water till the bottom of the small 

 inverted pan is covered. Fill a glass jar with water, 

 cover it with a glass plate, and invert it over the hole 

 in the bottom of the smaller pan, withdrawing the glass 

 plate under water. Now when the delivery tube is in 

 place, as shown in Figure i, apply heat to the mercuric 

 oxide in the tube, and the oxygen will be separated 

 from the mercury. The oxygen passes through the 

 delivery tube, and bubbles of it arising through the 

 water in the jar displace the water. This method of 

 collecting a gas is called collecting over water. When the 

 water is all displaced, the jar is full of oxygen (O) ; 

 a piece of glass or the hand may be placed under the 

 jar and the jar placed upright. 



The reaction is represented as follows : 



Another more common method of procuring oxygen 

 for experimental purposes is to obtain it from potassium 

 chlorate (KC1O 3 ). Use the same apparatus as that 

 just described. About one tablespoonful of KC1O 3 

 mixed with an equal quantity of manganese dioxide 

 (MnO 2 ) may be placed in the ignition tube and heated 

 as before. The manganese dioxide does not give up its 



