188 CORRELATION OF PHYSICAL FORCES. 



Whether the regarding electricity, light, magnetism, &c., 

 as simply motions of ordinary matter, be or be not admissi 

 ble, certain it is, that all past theories have resolved, and all 

 existing theories do resolve, the actions of these forces into 

 motion. Whether it be that, on account of our familiarity 

 with motion, we refer other affections to it, as to a language 

 which is most easily construed and most capable of explain 

 ing them ; whether it be that it is in reality the only mode in 

 which our minds, as contradistinguished from our senses, are 

 able to conceive material agencies ; certain it is, that since 

 the period at which the mystic notions of spiritual or preter 

 natural powers were applied to account for physical phenom 

 ena, all hypotheses framed to explain them have resolved 

 them into motion. Take, for example, the theories of light 

 to which I have before alluded : one of these supposes light 

 to be a highly rare matter, emitted from i. e. put in motion 

 by luminous bodies ; a second supposes that the matter is 

 not emitted from luminous bodies, but that it is put into a 

 state of vibration or undulation, i. e. motion, by them ; and 

 thirdly, light may be regarded as an undulation or motion of 

 ordinary matter, and propagated by undulation of air, glass, 

 &c., as I have before stated. In all these hypotheses, matter 

 and motion are the only conceptions. Nor, if we accept 

 terms derived from our own sensations, the which sensations 

 themselves may be but modes of motion in the nervous fila 

 ments, can we find words to describe phenomena other than 

 those expressive of matter and motion. We in vain struggle 

 to escape from these ideas ; if we ever do so, our mental 

 powers must undergo a change of which at present we see 

 no prospect. 



If we apply to any other force the mode of reasoning 

 which we have applied to heat, we shall arrive at the same 

 conclusion, and see that a given source of power can, sup 

 posing it to be fully utilised in each case, yield no more by 

 employing it as ah exciter of one force than of another. Let 



