536 SPECIAL METHODS OF DISCOVEKY. 



they found that an acid appeared at the positive wire 

 and an alkali at the .negative one ; and thus laid the 

 foundation of electro-chemistry and of the art of electro- 

 metallurgy. The true source of this acid and alkali was 

 subsequently discovered in the year 1806 by Sir H. Davy, 1 

 who by means of experiments, also found that the colour 

 of the electric light in rarefied media depends chiefly 

 upon some properties of the traces of ponderable matter 

 left in the vessels. When he passed the spark through a 

 vacuum over mercury, and gradually admitted air, the 

 colour was first green, then sea-green, blue, and purple 

 in succession, with increasing amounts of air ; and in a 

 vacuum over a melted fusible alloy of tin and bismuth, it 

 was yellowish and very pale. 



It was by means of study, hypotheses, and repeated 

 experiments, that Oersted at last succeeded, in the year 

 1819, in making the grand discovery of electro-magnetism. 

 Having a voltaic battery in action at one of his lectures, 

 he directed his assistant Hansteen to try the effect of its 

 current, when parallel to and near a delicately suspended 

 magnetic needle, and he was surprised to observe the needle 

 move. By means of additional experiments, he found 

 that it always moved in such a way as to tend to place 

 itself at right angles to the current. The result was pub- 

 lished in the year 1820. Ampere studied Oersted's results 

 deeply, and, by means of further experiments, found that 

 the poles of a freely suspended magnetic needle always 

 turn in one given direction with respect to the direction 

 of the current ; the direction being such that, supposing 

 the current to flow downwards through our body, if the 

 magnet is supported horizontally in front of us, its north- 

 seeking pole would place itself towards our right hand; and 

 that whatever change was made in the position of the con- 

 1 See page 432. 



