STUDY OF THE ACTIVE PART OF THE AIR 127 



Other ways of obtaining the active part of air. Quicksilver, 

 or mercury, when strongly heated in the air slowly combines with the 

 active part, and gradually becomes converted into a bright red powder 

 which is the calx of mercury. If some of this calx of mercury is heated 

 in a hard glass tube, as in Fig. 94, it soon changes in colour ; and as 

 the heating is continued it is noticed that a mirror-like deposit is formed 

 round the top, cold part of the tube. When this deposit is rubbed 

 with a penholder or pencil, it runs together and forms little drops of 

 quicksilver. Moreover, if a glowing splinter of wood is introduced 

 into the tube it bursts into flame, showing that the active part of the 

 air is being driven out of the red mercury rust. This change is just 

 the reverse of what takes place when mercury itself is heated. The 

 active part of the air, with which hot mercury slowly combines, is 

 driven out of the red mercury rust when that is strongly heated. But 

 red mercury calx is expensive, and it is too costly a plan to heat it to 

 obtain a quantity of the active part of air sufficient for the study of 

 its properties. Several other and cheaper substances easily give up 

 the active part of air when heated. 



The active part of the air is called oxygen. As it will be more 

 convenient in the future to speak of the active part of the air by the 

 name chemists use for it, it may be stated here that it is always called 

 oxygen, but the meaning of this name will be better understood later. 



58. PREPARATION OF OXYGEN. 



i. Oxygen from potassium chlorate. Place a little potassium chlorate 

 or chlorate of potash (which is the same thing) in a test-tube, and heat ifc 

 as in Fig. 94. Observe that the powder crackles, melts and gives off a 

 gas. Teat by a glowing splinter of wood, and see that the gas behaves, 

 like oxygen, the active part of the air. 



ii. Preparation of a small quantity 

 of oxygen. Powder some crystals 

 of potassium chlorate, and mix the 

 powder with a little manganese di- 

 oxide (sometimes called pyrolusite). 

 Heat some of themixtureinatest-tube 

 as in the last experiment. Observe 

 by putting in a glowing splinter that 

 oxygen is given off. Notice that 

 in this case there is no melting, and 

 the gas comes off more readily. 



iii. Preparation and collection of 

 oxygen. Into a hard glass tube, 

 closed at one end, fit an india-ruober . FIG. 95. Preparation of oxygen by heat- 

 _4.,___- .,1 11 , ,1 i ing a mixture of potassium chlorate and 



stopper, with one hole in it, through m * nganese dioxide. The gas is being 

 which a tube, bent as in Fig. 9o, is collected over water. 



