274 INDUCTION. 



periment; we may either ^?ic? an instance in natui'e suited to our purposes, 

 or, by an artificial arrangement of circumstances, make one. The value of 

 the instance depends on what it is in itself, not on the mode in which it is 

 obtained : its employment for the purposes of induction depends on the 

 same jDrinciples in the one case and in the other; as the uses of money are 

 the same whether it is inherited or acquired. There is, in short, no differ- 

 ence in kind, no real logical distinction, between the two processes of in- 

 vestigation. There are, however, practical distinctions to which it is of 

 considerable importance to advert. 



§ 3. The first and most obvious distinction between Observation and 

 Experiment is, that the latter is an immense extension of the former. It 

 not only enables us to produce a much greater number of variations in the 

 circumstances than nature spontaneously offers, but also, in thousands of 

 cases, to produce the precise sort of variation which we are in want of for 

 discovering the law of the phenomenon ; a service which nature, being con- 

 structed on a quite different scheme from that of facilitating our studies, 

 is seldom so friendly as to bestow upon us. P'or example, in order to as- 

 certain 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 ar- 

 tificial experiment for our knowledge that it is the former, and not the lat- 

 ter, which supports respiration ; and for our knowledge of the very exist- 

 ence of the two ingredients. 



Thus far the advantage of experimentation over simple observation is 

 universally recognized : all are aware that it enables us to obtain immmer- 

 able combinations of circumstances which arc 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, an- 

 other prerogative) of instances artificially obtained over spontaneous in- 

 stances — 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 

 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 presence 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 man- 

 kind 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 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, there- 



