252 ELEMENTARY GENERAL SCIENCE CHAP. 



EXPT. 259. Place a little copper in a test-tube with some 

 strong sulphuric acid. There is no action. Heat the tube, 

 and observe the smell of sulphur dioxide. 



EXPT. 260. In a flask fitted with a cork and delivery tube 

 place some copper turnings and some strong sulphuric acid, 

 and heat over wire gauze by means of a Bunsen flame. As 

 the gas is soluble in water, it cannot be collected in the same 

 way as hydrogen and oxygen. Being heavier than air, how- 

 ever, it may be collected in the manner shown in the diagram 

 (Fig. 128). Obtain by this means two jars of the gas. 



In (1) place a lighted taper, and in (2) a piece of moist red 

 flannel or a coloured flower, allowing it to remain for some 

 time. 



Properties of Sulphur Dioxide. By these experiments you 

 will have learnt that the gas does not burn, does not support 

 combustion, and that it has the power of bleaching vegetable 

 colours. This important property is largely made use of in 

 technology, the gas being employed for bleaching, especially for 

 delicate materials such as straw, silk, &c. 



Sulphurous Acid and Sulphites. EXPT. 261. Using the 

 flask you have already fitted for the preparation of the gas, allow 

 the sulphur dioxide to bubble through a solution of caustic 

 soda, which, we have already seen, has the power of turning 

 red litmus blue, and is called an alkali. Observe that the gas 

 is absorbed, and that after a time the liquid smells of the gas 

 and has become slightly acid. Boil the solution and observe 

 that the smell disappears, showing that the gas is expelled by 

 heating, while by further evaporation a white, solid is ulti- 

 mately left. Examine this solid, which is evidently soluble 

 in water, and satisfy yourself that it is not caustic soda, having 

 110 action on litmus. Add to it a little sulphuric acid and ob- 

 serve the smell of sulphur dioxide, indicating that this was 

 previously contained in the solid. 



The formation of this solid is an example of the production of 

 a salt, and we shall find later that many other salts are formed by 

 analogous methods. The salts produced by neutralising sulphur- 

 ous acid, as the solution of sulphur dioxide is called, are known 

 as sulphites, and the particular salt we have thus prepared is 

 sodium sulphite. All the sulphites are characterised by their 

 property of giving off sulphur dioxide when acted upon by sul- 

 phuric or other strong acid. 





