180 GENERAL CONDITIONS OF SCIENTIFIC RESEARCH. 



Oersted's great discovery of electro-magnetism, made 

 in the year 1819, was also, prelumined in a similar manner. 

 It had long been known that lightning, by passing through 

 the magnetic needles of ships' compasses, destroyed, in- 

 jured, or reversed their polarity, and in some rare instances 

 caused them to point east and west instead of north and 

 south ; and by passing through steel knives or rods, some- 

 times made them strongly magnetic. In 1676, Grrofton, 

 a master of a ship, observed that a violent thunderstorm 

 reversed the polarity of his compass-needles. 1 About the 

 year 1777 Beccaria 'noticed that a needle through which 

 he had sent an electric shock, had in consequence acquired 

 a curious species of polarity ; for, instead of turning as 

 usual to the north and south, it assumed a position at right 

 angles to this, its two ends pointing to the east and west.' 2 

 Romagnosi also, about the year 1805, observed that a 

 magnetised needle ' experiences a declination whilst under 

 the influence of a voltaic current ; ' and about the same time 

 M. Mojon noticed that unmagnetised needles when sub- 

 jected to such a current 6 acquire by this means a kind of 

 magnetic polarity.' 3 



In other cases, the starting-point of a great discovery 

 has been an unsubstantial hypothesis ; for instance, the 

 hypothesis of Avogadro, that equal volumes of all gases 

 contain equal numbers of molecules, was advanced in the 

 year 1811 with but little evidence to support it; but the 

 important discovery made by Gay Lussac in the year 1814, 

 that gases combine together in simple proportions by 

 volume, largely confirmed it, and it has now become an 

 important fundamental principle of physical and chemical 



1 Philosophical Transactions of the Royal Society, 1676, p. 647. 



2 Eoget's < Treatise on Electro-Magnetism,' p. 3, in Library of Use- 

 ful Knowledge, 1832. 



3 Manuel du Galvanisme, par J. Izara (Paris, 1805), p. 120. 



