546 SPECIAL METHODS OF DISCOVERY. 



Stokes, W. A. Miller, Angstrom, Thaleu, Swan, Bunsen, 

 Kirchoff, Lockyer, Janssen, and others, 1 by extending the 

 investigation of spectral lines originally discovered by Wol- 

 laston, evolved the entire science of spectroscopic analysis. 

 c. By using known instruments or forces in a new 

 way. Sometimes even a slight variation of a known 

 experiment is attended by a different effect, and leads 

 to a discovery. By the employment of the prism in a 

 novel manner, Newton was enabled to make his memo- 

 rable discovery of the composition and dispersion of white 

 light ; and nearly all the discoveries which have since 

 been made in spectrum analysis, and which now constitute 

 a very important branch of physical science, were effected 

 by a similar method. In the year 1775, Graham suggested 

 the employment of the magnetic pendulum, or needle of 

 oscillation, to discover the variations of magnetic intensity. 

 The use of the voltaic current also in a particular new 

 way led to the discovery of the alkali-metals by Davy, the 

 decomposition of water by Nicholson and Carlisle (on May 2, 

 ' 800), and to the whole of the discoveries which have since 

 been made in the science of electro-chemistry. ' Messrs. 

 Nicholson and Carlisle were the first persons who repeated 

 Volta's experiments with the voltaic apparatus, which 

 speedily drew the attention of all Europe. They ascertained 

 that the zinc end of the pile was positive, and the copper end 

 negative. Happening to put a drop of water on the upper- 

 most plate, and to put into it the extremity of a gold wire 

 connected with the undermost plate, they observed an 

 extrication of air-bubbles from the wire. This led them to 

 suspect that the water was decomposed. To determine the 

 point, they collected a little of the gas extricated, and 

 found it hydrogen. They then attached a gold wire to 

 the zinc end of the pile, and another gold wire to the 

 1 See pages 178, 179. 



