OBSERVATION AND EXPERIMENT. 443 



phenomena which nature exhibits on a grander scale 

 under the form of lightning and thunder. Now let 

 any one consider what amount of knowledge of the 

 effects and laws of electric agency mankind could 

 ever have obtained ^from the mere observation of 

 thunder-storms, and compare it with that which they 

 have gained, and may expect to gain, from electrical 

 and galvanic experiments. This example is the more 

 striking, now that we have reason to believe that elec- 

 tric action is of all natural phenomena (except heat) 

 the most pervading and universal, which, therefore, it 

 might antecedently have been supposed could stand 

 least in need of artificial means of production to 

 enable it to be studied while the fact is so much the 

 contrary, that without the electric machine, the voltaic 

 battery, and the Leyden jar, we should never have 

 suspected the existence of electricity as one of the 

 great agents in nature ; the few electric phenomena 

 we should have known of would have continued to be 

 regarded either as supernatural, or as a sort of anoma- 

 lies and eccentricities in the order of the universe. 



When we have succeeded in insulating the pheno- 

 menon which is the subject of inquiry, by placing it 

 among known circumstances, we may produce further 

 variations of circumstances to any extent, and of such 

 kinds as we think best calculated to bring the laws of 

 the phenomenon into a clear light. By introducing 

 one well defined circumstance after another into the 

 experiment, we obtain assurance of the manner in 

 which the phenomenon behaves under an indefinite 

 variety of possible circumstances. Thus, chemists, 

 after having obtained some newly-discovered sub- 

 stance in a pure state, (that is, having made sure that 

 there is nothing present which can interfere with and 

 modify its agency,) introduce various other substances, 



