THE EXACT MEASUREMENT OF PHENOMENA. 323 



from prepossession by another object : as one powerful 

 smell renders other smells in the same room imper- 

 ceptible ; from the interruption of interposing bodies : 

 as the internal parts of animals ; and because the object 

 is unfit to make an impression upon the sense : as the 

 air or the invisible and untangible spirit which is in- 

 cluded in every living body/ 



Complexity of Quantitative Questions. 



One remark which we may well make in entering 

 upon quantitative questions, has regard to the great variety 

 and extent of phenomena presented to our notice. So 

 long as we deal only with a simply logical question, that 

 question is merely, Does a certain event happen "? or, Does 

 a certain object exist "? No sooner do we regard the event 

 or object as capable of more or less, than one question 

 branches out into many. We must now ask, How much 

 is it compared with its cause or necessary condition 1 

 Does it change when the amount of the cause changes ? 

 If so, does it change in the same or opposite direction 1 Is 

 the change in simple proportion to that of the cause ? If 

 not, what more complex law of connection holds true \ 

 This law determined satisfactorily in one series of cir- 

 cumstances may be varied under new conditions, and the 

 most complex relations of several quantities may ultimately 

 be established. 



In every question of physical science there is thus a 

 series of steps of progress, the first one or two of which 

 are usually made with ease, while the succeeding ones 

 demand more and more careful measurement. We cannot 

 lay down any single invariable series of questions which 

 must be asked from nature. The exact character of the 

 questions will vary according to the nature of the case, 

 but they will usually be of a very evident kind, and we 

 may readily illustrate them by actual examples. Suppose, 



Y 2 



