DISCOVERY BY USING KNOWN MEANS IN A NEW WAY. 547 



copper end, and plunged the two wires into a glass of 

 water, taking care not to allow them to touch each other. 

 Gas was extricated from both wires. On collecting that 

 from the wire attached to the zinc end, it was found to be 

 oxygen gas, while that from the copper end was hydrogen 

 gas. The volume of hydrogen gas extricated was just 

 double that of the oxygen gas ; and the two gases being 

 mixed, and an electric spark passed through them, they 

 burnt with an explosion, and were completely converted 

 into water. Thus it was demonstrated that water was 

 decomposed by the action of the pile, and that the oxygen 

 was extricated from the positive pole and the hydrogen 

 from the negative. This held when the communicating 

 wires were gold or platinum ; but if they were of copper, 

 silver, iron, lead, tin, or zinc, then only hydrogen gas was 

 extricated from the negative wire. The positive wire 

 evolved little or no gas, but it was rapidly oxidised. 

 Thus the connection between chemical decompositions and 

 electrical currents was first established.' 1 By using the 

 voltaic current in a new manner, Grerboin, in the year 

 1801, discovered the electrolytic movements of mercury. 

 Trommsdorff also, about the- year. 1803, by employing very 

 large plates in a voltaic battery, was the first to discover 

 that thin leaves of metal might be ignited by means of 

 voltaic electricity. It was by using in a particular manner 

 the conductor of a voltaic current that Oersted discovered 

 electro-magnetism, and Ampere discovered the laws of 

 electro-magnetic action. 



In the production of colours from white light by means 

 of thin films, Newton, by adopting the device of pressing 

 two glass lenses together, was enabled to discover the 

 thickness of the film which was necessary for the produc- 

 tion of each colour. By examining the different kinds 

 1 Thomson, History of Chemistry, vol. ii. p. 255. 



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