CHLOKINE. 169 



heating potassium chlorate, the chemical action is one of simple 

 decomposition. Potassium chlorate is really a compound of oxygen 

 with another substance termed potassium chloride, and when heated 

 breaks up into these two constituents, the former escaping as a gas, 

 whilst the latter remains as a fused mass in the test-tube. Many 

 other compounds containing oxygen behave in similar fashion 

 when heated ; i.e., they break up, evolving oxygen and some other 

 substance which together with oxygen form the constituents of 

 the compounds. 



Place in a test-tube as much red oxide of mercury as will lie on a 

 sixpence, and heat it pretty strongly in the Bunsen lamp ; the red 

 powder will first turn almost black, and will then become decom- 

 posed by the heat ; oxygen gas will be liberated, as may be shown 

 by introducing into the mouth of the tube a glowing spill of wood 

 (Expt. 176), which will burst into flame; simultaneously small 

 globules of quicksilver will condense on the side of the tube. Let 

 the tube cool, and then introduce a pencil or glass rod, and rub 

 together the film of condensed mercury therewith, when you will 

 see that it actually does consist of globules of quicksilver. 



For experiments illustrating the chemical properties of oxygen, 

 vide Chapter XIV. 



Expt. 179. To prepare Chlorine Gas by heating a Solid. 

 Into a test-tube introduce one or two grains of solid chloride of 

 gold, and apply heat; decomposition will ensue, and chlorine gas will 

 be evolved, whilst metallic gold remains in the tube in a " spongy " 

 state, i.e., in somewhat loose particles. To test the production of 

 chlorine, the fumes evolved at the mouth of the tube may be 

 cautiously wafted towards the nose, when the peculiar smell of 

 diluted chlorine will be smelt ; care should be taken not to take a 

 strong sniff directly at the mouth of the tube, on account of the 

 choking and irritating properties of chlorine when breathed in any 

 quantity. (Compare Expt. 162.) 



Another test for chlorine is its property of turning blue a 

 mixture of potassium iodide and starch (Expt. 164). Prepare a 

 mixture of starch paste and potassium iodide, dip a piece of white 

 paper therein, and place the paper over the mouth of the test-tube ; 

 the chlorine passing off will turn the paper blue where it comes in 

 contact with it. 



Expt. 180. To prepare Chlorine in larger Quantity. The 

 ordinary method of preparing chlorine gas it to heat strong hydro- 

 chloric acid solution with powdered manganese dioxide, using the 

 arrangement indicated by fig. 55. The gas should be collected by 

 displacement in jars held mouth upwards, as the gas is heavier 

 than air ; unless a properly constructed draught cupboard (Expt. 



