THE FOUR EXPERIMENTAL METHODS. 437 



which we had not already calculated and subducted the effect. 

 But as we can never be quite certain of this, the evidence de 

 rived from the Method of Eesidues is not complete unless we 

 can obtain C artificially and try it separately, or unless its 

 agency, when once suggested, can be accounted for, and proved 

 deductively from known laws. 



Even with these reservations, the Method of Residues is 

 one of the most important among our instruments of dis 

 covery. Of all the methods of investigating laws of nature, 

 this is the most fertile in unexpected results ; often informing 

 us of sequences in which neither the cause nor the effect were 

 sufficiently conspicuous to attract of themselves the attention 

 of observers. The agent C may be an obscure circumstance, 

 not likely to have been perceived unless sought for, nor likely 

 to have been sought for until attention had been awakened by 

 the insufficiency of the obvious causes to account for the whole 

 of the effect. And c may be so disguised by its intermixture 

 with a and b, that it would scarcely have presented itself 

 spontaneously as a subject of separate study. Of these uses of 

 the method, we shall presently cite some remarkable examples. 

 The canon of the Method of Residues is as follows : 



FOURTH CANON. 



Subduct from any phenomenon such part as is knoivn by pre 

 vious inductions to be the effect of certain antecedents, and the 

 residue of the phenomenon is the effect of the remaining ante 

 cedents. 



6. There remains a class of laws which it is imprac 

 ticable to ascertain by any of the three methods which I have 

 attempted to characterize ; namely, the laws of those Permanent- 

 Causes, or indestructible natural agents, which it is impossible 

 either to exclude or to isolate ; which we can neither hinder 

 from being present, nor contrive that they shall be present 

 alone. It would appear at first sight that we could by no means 

 separate the effects of these agents from the effects of those 

 other phenomena with which they cannot be prevented from 

 coexisting. In respect, indeed, to most of the permanent 

 causes, no such difficulty exists ; since though we cannot 



