4-18 INDUCTION. 



in each component element of the atmosphere separately. But 

 nature does not supply either oxygen or azote in a separate 

 state. We are indebted to artificial experiment for our know 

 ledge that it is the former, and not the latter, which supports 

 respiration ; and for our knowledge of the very existence of the 

 two ingredients. 



Thus far the advantage of experimentation over simple ob 

 servation is universally recognised : all are aware that it enables 

 us to obtain innumerable combinations of circumstances which 

 are not to be found in nature, and so add to nature s experi 

 ments a multitude of experiments of our own. But there is 

 another superiority (or, as Bacon would have expressed it, 

 another prerogative) of instances artificially obtained over 

 spontaneous instances, of our own experiments over even the 

 same experiments when made by nature, which is not of less 

 importance, and which is far from being felt and acknowledged 

 in the same degree. 



When we can produce a phenomenon artificially, we can 

 take it, as it were, home with us, and observe it in the midst 

 of circumstances with which in all other respects we are accu 

 rately acquainted. If we desire to know what are the effects 

 of the cause A, and are able to produce A by means at our 

 disposal, we can generally determine at our own discretion, so 

 far as is compatible with the nature of the phenomenon A, the 

 whole of the circumstances which shall be present along with 

 it : and thus, knowing exactly the simultaneous state of every 

 thing else which is within the reach of A s influence, we have 

 only to observe what alteration is made in that state by the pre 

 sence of A. 



For example, by the electric machine we can produce 

 in the midst of known circumstances, the phenomena which 

 nature exhibits on a grander scale in 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 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 experi 

 ments. This example is the more striking, now that we have 



