442 INDUCTION. 



sphere separately. But nature does not supply either 

 oxygen or azote in a separate state. We are indebted 

 to artificial experiment for our knowledge that it is 

 the former, and not the latter, which supports respi- 

 ration; and even for our knowledge of the very 

 existence of the two ingredients. 



Thus far the advantage of experimentation over 

 simple observation is universally recognised : all are 

 aware that it enables us to obtain innumerable combi- 

 nations of circumstances which are not to be found in 

 nature, and so add to nature's experiments 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 accurately acquainted. If we 

 desire to know what are the effects of the cause A, 

 and are able to produce A by any means at our dis- 

 posal, we can generally determine at our own discre- 

 tion, 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 everything else 

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

 only to observe what alteration is made in that state 

 by the presence of A. 



For example, by the electrical machine we can 

 produce in the midst of known circumstances, the 



