680 DECOMPOSITION OF WATER. 



is then pressed well in, and finally, with the help of the blowpipe, 

 covered with sealing-wax, in the manner previously explained. 



In handling the potash solution care should be taken that none 

 of it drops on the hands or clothes, as it corrodes both the skin and 

 woollen cloth. If any of it should have dropped upon the clothes, 

 the part is first well washed with water and then moistened with 

 vinegar, which may afterwards be also washed out. 



The oxygen disengaged under these circumstances does not attack 

 the iron electrode ; although, if sulphuric acid were used instead of 

 potash, the iron would be rapidly dissolved, just as zinc is in 

 dilute sulphuric acid. The potash solution has the disadvantage ot 

 frothing, and hence some of it gets carried over into the dish with 

 the soap- water ; but this does not in any way interfere with the suc- 

 cess of the experiment. The cork is also attacked strongly by the 

 potash and gradually destroyed ; it must therefore be renewed from 

 time to time. 



The little glass tube should dip into the soap-water, no india- 

 rubber tubing being required ; it must of course be removed, with 

 the whole apparatus, before the oxy.hydrogen bubbles are lighted. 



Persons with sensitive ears should open their mouths when the 

 match is applied to the bubbles ; the sound will then have less effect 

 upon the ear. 



In the decomposition by the galvanic current of the 

 compounds of the heavy metals (gold, silver, copper, 

 etc.), the metal is always disengaged at the cathode, 

 while the substance or compound combined with the 

 metal is set free at the anode. Thus if a current passes 

 through an aqueous, solution of cupric sulphate (blue 

 vitriol, see page 215), the cathode is soon covered with 

 a bright layer of pure copper, while the sulphuric acid 

 which was in combination with the copper is disengaged 

 at the anode. 



The platinum strips used for the decomposition of sodic sulphate 

 are used as electrodes in this experiment also. They are dipped 

 into a small jar or beaker, which contains the cupric sulphate solu- 

 tion, and are kept about 1 or 2 cm apart. The red copper deposit 

 upon the cathode cannot be seen until the platinum strip is taken 

 out of the blue liquid. When an appreciable layer of copper has 

 been deposited the electrodes are interchanged, that is, the cathode 



