220 ELEMENTARY GENERAL SCIENCE CHAP. 



which occur when this rust is heated should be carefully de- 

 scribed in a notebook. 



EXPT. 213. Place some of the mercury rust (known as red 

 oxide of mercury) in a tube of hard glass closed at one end, 

 and heat strongly. Notice the darkening of the powder, also 

 the dark deposit which collects round the inside of the tube 

 above the powder. Place into the tube a splinter of wood 

 which has been just extinguished and is still glowing. Note 

 that it glows more brightly, or even bursts into flame. Allow 

 the tube to cool, and notice that the powder returns to its 

 original colour. With a piece of wood or glass scrape off the 

 dark deposit from the walls of the tube. It is seen to be bright 

 metallic quicksilver or mercury. 



What has this experiment taught ? It has shown that by 

 heating the rust of mercury we obtain mercury itself, and a gas 

 in which wood burns more brightly than in air. Is this what we 

 should expect to find supposing we have obtained the active pa rt 

 which we hoped to get ? It certainly is, for we have found that 

 the gas occupying the other four-fifths of the air does not support 

 combustion at all, and thus acts as a diluent. Consequently the 

 active part itself would be expected to support combustion very 

 vigorously. 



Absolute proof is, however, forthcoming. It may be proved, 

 by weighing, that the weight of the original mercury is equal to 

 that left after the experiment, provided that all the rust is decom- 

 posed and no mercury is lost. This shows that the gas escaping 

 from the rust is the same gas as that taken from the air. This 

 gas is called Oxygen, and the change may be thus stated : Oxide 

 of mercury, when heated, decomposes into mercury and 

 oxygen. 



Preparation and Properties of Oxygen. As the quantity of 

 oxygen obtained by the above method is comparatively small, 

 and the oxide is expensive, a more convenient source of the gas 

 is a white crystalline powder called Potassium Chlorate. 



EXPT. 214. Place a little potassium chlorate in a test-tube 

 and heat. Observe that the mass crackles, melts, and gives 

 off a gas. Test by a glowing match, and see that the gas is 

 oxygen. The gas is, however, given off more readily, and 

 without fusion, if a little manganese dioxide is added to the 

 chlorate. By this means we can collect some jars of the gas. 



