674 CHEMICAL EFFECTS OF THE CURRENT. 



current decomposes it again into these two constituents. 

 There are some blue vegetable colours which are turned 

 red by sulphuric acid, and green by soda, while the 

 blue is not altered at all by Glauber's salt. If a solu- 

 tion of Glauber's salt is coloured blue by any of these 

 colouring matters, and two strips of platinum foil, 

 connected with the terminals of a battery of at least 

 two strong elements, be dipped into the solution, 

 sulphuric acid will be given off at the strip which 

 forms the anode, and soda at the cathode, hence the 

 blue colour will change into red at the anode and 

 into green at the cathode. If the electrodes are with- 

 drawn from the liquid, and the whole well stirred, the 

 two substances which have been separated combine 

 again, and the blue colour is restored. 



The colouring matter in most blue flowers for example, the 

 violet, lobelia, the common flag of our gardens (iris germanica), etc., 

 is changed to red by sulphuric acid, and to green by soda. To obtain 

 the colouring matter some of the flowers should be digested with a 

 small quantity of warm alcohol ; the alcohol is then poured off, and 

 the flowers pressed in muslin. A blue solution is obtained, which 

 Avill yield a blue pigment if gently evaporated. The blue pigment 

 dissolves in water and may be used for giving a blue colour to the 

 solution of Glauber's salt. An easier plan is to make a decoction of 

 the leaves of red cabbage. A few leaves are cut up small, just covered 

 with water, and heated until the water begins to boil. The liquid 

 is poured off, filtered, and a little Glauber's salt dissolved in it 

 (about 5 grammes in 50 CC ). The solution obtained in this way is 

 usually rather red, but the necessary blue colouring may be given 

 to it by the addition of a minute quantity of soda. A piece of soda 

 of the size of a pea is dissolved in a spoonful of water, and drop 

 after drop of it poured into the red liquid until it becomes bluo. 

 The liquid must be stirred after each drop before another is added. 

 An excess of soda must be most carefully avoided ; let the blue 

 liquid have a violet rather than a green tinge. 



The effect of the sulphuric acid and soda upon the blue colour 

 may easily be seen by pouring a small quantity of the blue solution 



