250 



INDUCTION. 



variation which we are in want of for 

 discovering the law of the pheno- 

 menon, — a service which nature, 

 being constructed on a quite different 

 scheme from that of facilitating our 

 studies, i« seldom so friendly as to 

 bestow upon us. Yor example, in 

 order to ascertain what principle in 

 the atmosphere enables it to sustain 

 life, the variation we require is that 

 a living animal should be immersed 

 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 experi- 

 mentation over simple observation is 

 universally recognised : all are aware 

 that it enables us to obtain innume- 

 rable combinations 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 arti- 

 ficially 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 pheno- 

 menon 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 means at our disposal, we can 

 generally determine at our own dis- 

 cretion, 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 simulta- 

 neous state of everything else which 



is within the reach of A's influence, 

 we have only to observe what altera- 

 tion 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 thunderstorms, 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 electric 

 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 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 continued to be re- 

 garded either as supernatural, or as 

 a sort of anomalies and eccentricities 

 in the order of the universe. 



When we have succeeded in insu- 

 lating 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 ob- 

 tain assurance of the manner in which 

 the phenomenon behaves under an 

 indefinite variety of possible circum- 

 stances. Thiis, chemists, after having 

 obtained some newly-discovered sub- 

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



