154? DISCOURSE ON THE STUDY 



conclude that our sensation of the different pitches 

 of musical notes originates in the different rapidities 

 with which their impulses are communicated to our 

 ears. 



(154.) 6th, That such counteracting or modifying 

 causes may subsist unperceived, and annul the 

 effects of the cause we seek, in instances which, 

 but for their action, would have come into our class 

 of favourable facts ; and that, therefore, exceptions 

 may often be made to disappear by removing or 

 allowing for such counteracting causes. This remark 

 becomes of the greatest importance, when (as is 

 often the case) a single striking exception stands 

 out, as it were, against an otherwise unanimous array 

 of facts in favour of a certain cause. 



(155.) Thus, in chemistry, the alkaline quality of 

 the alkaline and earthy bases is found to be due to 

 the presence of oxygen combined with one or other 

 of a peculiar set of metals. Ammonia is, however, 

 a violent outstanding exception, such as here alluded 

 to, being a compound of azote and hydrogen : but 

 there are almost certain indications that this excep- 

 tion is not a real one, but assumes that appearance 

 in consequence of some modifying cause not under- 

 stood. 



(156.) 7th, If we can either find produced by 

 nature, or produce designedly for ourselves, two in- 

 stances which agree exactly in all but one particular, 

 and differ in that one, its influence in producing 

 the phenomenon, if it have any, must thereby be 

 rendered sensible. If that particular be present 

 in one instance and wanting altogether in the 

 other, the production or non-production of the phe- 



