OBSERVATION AND EXPERIMENT. 419 



reason to believe that electric action is of all natural pheno 

 mena (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 Leyden jar, and the voltaic 

 battery, we probably 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 con 

 tinued to be regarded either as supernatural, or as a sort of 

 anomalies and eccentricities in the order of the universe. 



^When we have succeeded in insulating the phenomenon 

 which is the subject of inquiry, by placing it among known 

 circumstances, we may produce further variations of circum 

 stances 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 substance 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, one by one, to ascertain whether it 

 will combine with them, or decompose them, and with what 

 result; and also apply heat, or electricity, or pressure, to dis 

 cover what will happen to the substance under each of these 

 circumstances. 



But if, on the other hand, it is out of our power to pro 

 duce the phenomenon, and we have to seek for instances in 

 which nature produces it, the task before us is very different. 

 Instead of being able to choose what the concomitant cir 

 cumstances shall be, we now have to discover what they are ; 

 which, when we go beyond the simplest and most accessible 

 cases, it is next to impossible to do, with any precision and 

 completeness. Let us take, as an exemplification of a phe 

 nomenon which we have no means of fabricating artificially, 

 a human mind. Nature produces many; but the consequence 



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