CORRELATION OF NERVOUS AND MENTAL FORCES. XXX111 



the one hand than nerve-force is removed from motor force on the 

 other. Each, in giving origin to the next, is itself expended or 

 ceases to exist as such, and each bears, in its own intensity, a pre 

 cise relation to that of its antecedent and its consequent.&quot; &quot;We have 

 here only space briefly to trace the principle in its application to 

 sensations, motions, and intellectual operations. 



. The physical agencies acting upon inanimate objects in the 

 external world, change their form and state, and we regard these 

 changes as transformed manifestations of the forces in action. A 

 body is heated by hammering ; the heat is but transmuted mechani 

 cal force; or a body is put in motion by heat, a certain quantity 

 being transformed into mechanical effect, or motion of the mass. 

 And so it is held that no force can arise except by tjie expenditure 

 of a preexisting force. Now, the living system is acted upon by 

 the same agencies and under the same law. Impressions made 

 upon the organs of sense give rise to sensations, and we have the 

 same warrant in this, as in the former case, for regarding the effects 

 as transformations of the forces in action. If the change of 

 molecular state in a melted body represents the heat transformed 

 in fusing it, so the sensation of warmth in a living body must 

 represent the heat transformed in producing it. The impression on 

 the retina, as well as that on the -photographic tablet, results from 

 the transmuted impulses of light. And thus impressions made 

 from moment to moment on all our organs of sense, are directly 

 correlated with external physical forces. This correlation, further 

 more, is quantitative as well as qualitative. Not only does the 

 light-force produce its peculiar sensations, but the intensity of these 

 sensations corresponds with the intensity of the force ; not only is 

 atmospheric vibration transmuted into the sense of sound, but the 

 energy of the vibration determines its loudness. And so in all 

 other cases ; the quantity of sensation depends upon the quantity 

 of the force acting to produce it. 



Moreover, sensations do not terminate in themselves, or come 

 to nothing; they produce certain correlated and equivalent effects. 



