r6 CORRELATION OF PHYSICAL FORCES. 



progression. A similar deduction convinces us of the motion 

 of electricity. 



As we in common parlance speak of sound moving, although 

 sound is motion, it requires no great stretch of imagination to 

 conceive light and electricity as motions, and not as things 

 of moving. If one end of a long bar metal be struck, a sound 

 is soon perceptible at the other end. This we now know to be 

 a vibration of the bar ; sound is but a word expressive of the 

 mode of motion impressed on the bar ; so one end of a column 

 of air or glass subjected to a luminous impulse gives a per- 

 ceptible effect of light at the other end : this can equally be 

 conceived to be a vibration or transmitted motion of particles 

 in the transparent column : this question will, however, be 

 further discussed hereafter ; for the present we will confine 

 ourselves to motion within the limits to which the term is 

 usually restricted. 



With the perceptible phenomena of motion the mental 

 conception has been invariably associated to which I have 

 before alluded, and to which the term force is given the 

 which conception, when we analyse it, refers us to some antece- 

 dent motion. If we except the production of motion by heat, 

 light, &c., which will be considered in the sequel, when w r e see 

 a body moving we look to motion having been communicated 

 to it by matter which has previously moved. 



Of absolute rest Nature gives us no evidence : all matter, 

 as far as we can ascertain, is ever in movement, not merely in 

 masses, as with the planetary spheres, but also molecularly, 

 or throughout its most intimate structure : thus every altera- 

 tion of temperature produces a molecular change throughout 

 the whole substance heated or cooled ; slow chemical or elec- 

 trical actions, actions of light or invisible radiant forces, are 

 always at play, so that as a fact we cannot predicate of any 

 portion of matter that it is absolutely at rest. Supposing, 

 however, that motion is not an indispensable function of 

 matter, but -that matter can be at rest, matter at rest would 

 never of itself cease to be at rest ; it would not move unless 

 impelled to such motion by some other moving body, or body 

 which has moved. This proposition applies not merely to 

 impulsive motion, as when a ball at rest is struck by a moving 



