DISCOVERY BY COMPARING FACTS AND GENERALISING. 581 



liquids, gases, metals, non metals, elements, compounds, 

 soluble and insoluble bodies ; combustible and incom- 

 bustible substances ; supporters and non-supporters of life ; 

 elements and compounds ; conductors and non-conductors 

 of beat and of electricity ; positive and negative bodies ; 

 magnetic and diamagnetic substances ; animate and in- 

 animate bodies ; attractive and repulsive forces ; common 

 and rare properties or actions ; abstract and concrete phe- 

 nomena, &c. &c. 



Discovery by generalisation, or simply classing together 

 similar substances or phenomena, has constituted a very 

 necessary step in the evolution of knowledge, but is fast 

 being replaced by the more truthful method of classi- 

 fication (or rather arrangement) of bodies and actions in 

 natural series, in accordance with their relations of mutual 

 dependence as cause and effect. 



d. By comparing collections of facts with each other. 

 This is one of the methods by means of which we discover 

 what is usually termed ' general truths.' In this way it 

 has been found that bases are electro-positive and acids 

 electro-negative ; that conductors of electricity are fre- 

 quently also conductors of heat ; that electro-positive 

 substances are usually combustible ; that good conductors 

 of heat are commonly metallic, &c. By classing together 

 all crystals which, like tourmaline, became electric by heat, 

 and all those which were hemihedric, Haiiy observed 

 that the two classes were identical, and thus discovered a 

 connection between the electric properties and molecular 

 structure of crystals. It was also by classing together all 

 crystals which expanded equally in all directions by heat, 

 the important discovery was made that they belonged to 

 the cubical or monometric system only. 



Sir David Brewster discovered, in the year 1818, by 

 the optical examination and comparison of crystals, a 



